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Old 07-01-2012, 16:29   #1
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Polishing Linear Polyurethane Imperfections

Help! I just painted my boat with linear polyurethane (Interthane 990) and am disappointed with the finish. Had there been a commercial sprayer available I would have gone that route but, alas, there was none so I did it myself. Parts came out excellent and parts with orange peel and runs. Can those be sanded and polished, and how? If it can what grits do I start and finish with? Any other tips/techniques? It's not a paint like awlgrip that stratifies and there's plenty of paint on.
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Old 07-01-2012, 16:53   #2
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Re: Polishing linear polyurethane imperfections?

Pigments in LPU are in the surface, not suposed to be sand the final coat. If you are looking for perfection, start by wet sanding by hand in a circular motion with 320 grit on all the runs and high spots ujntil level, then wet sand the whole boat and apply another coat. Learning curve with LPU's is steep and rookies shouldn't expect superior results, especially if painting outdoors. Another option would be to prep as above and shoot with Awlcraft arcylic urethane, a couple of coats to build some mil thickness, this paint can be wet sanded, and polished on the final coat to remove imperfections.
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Old 07-01-2012, 17:11   #3
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Re: Polishing linear polyurethane imperfections?

Thanks for the reply. What I have on now is an acrylic urethane like Awlcraft, I presume. [I]Awlgrip[I] is a polyester urethane and has different layers when it cures. I've read that that can't be sanded/buffed. I agree about the difficulty in spray painting. Still, I want to improve the appearance. It's not clear to me that sanding and repainting (by me) will lead to better results. Should I let it cure before sanding/polishing or is it better to do it before it gets full hardness in several days?
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Old 07-01-2012, 17:26   #4
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Re: Polishing linear polyurethane imperfections?

I prefer NON marine specific urethanes like ameron coatings or Jotun

Far more affordable than awlgrip and sterling and tough enough for mining equipment and oilrigs, so certainly good enough for boats

Have seen them used on boats for over 20 years, easily buff able and re-coatable
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Old 07-01-2012, 17:33   #5
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Re: Polishing linear polyurethane imperfections?

I am not familiar with the brand of paint you used, but if it is an acrylic or one part urethane, it should be polishable -to some degree. Don't expect a "perfect finish, but remember that only you know where the imperfections are. First, yes, give it time to settle/ harden - at least a few weeks, a month is not too long. I would wet sand GENTLY the high spots (runs, and tops of orange peel with 400-600 grit). Use a very gentle touch and a sanding block/firm sponge here - your fingers will dig in without you realizing it. You only want to remove the high spots. Then polish with an electric/pneumatic polisher and good polish - something like 3M's 3-step system. The polish wont be cheap, but it is worth it. Follow with wax and you will likely be the only one who knows.

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Old 07-01-2012, 17:50   #6
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Re: Polishing linear polyurethane imperfections?

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Originally Posted by Epicurean View Post
Thanks for the reply. What I have on now is an acrylic urethane like Awlcraft, I presume. [I]Awlgrip[I] is a polyester urethane and has different layers when it cures. I've read that that can't be sanded/buffed. I agree about the difficulty in spray painting. Still, I want to improve the appearance. It's not clear to me that sanding and repainting (by me) will lead to better results. Should I let it cure before sanding/polishing or is it better to do it before it gets full hardness in several days?
Linear polyurethanes and acrylic urethanes are two distincly different products. In your first post you indicated LPU as such I replyed accordingly. If what you have is truly an acrylic urethane this product can indeed be wet sanded and polised, then waxed. You need to insure that you have sprayed enough coats as you will be removing paint and if to thin it is possible to sand through to the primer or subcoat. In the hands of someone who knows how to sand and polish this type of paint it is possible to get a near perfect result. Speak to the manufacturer of the product to get additional info on their paint. Good luck.
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Old 07-01-2012, 18:41   #7
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Re: Polishing linear polyurethane imperfections?

Quote:
Originally Posted by Epicurean View Post
Help! I just painted my boat with linear polyurethane (Interthane 990) and am disappointed with the finish. Had there been a commercial sprayer available I would have gone that route but, alas, there was none so I did it myself. Parts came out excellent and parts with orange peel and runs. Can those be sanded and polished, and how? If it can what grits do I start and finish with? Any other tips/techniques? It's not a paint like awlgrip that stratifies and there's plenty of paint on.

Interthane 990 is an acrylic urethane and as such will wet sand and polish just fine. I would suggest guide coating with a proper guide coat to help you not blow through, and then dry sand starting with 1000 grit film discs on a DA, then 1500, then polish. You can go up to 2000 grit wet/dry film discs for the DA, but you may remove too much material and start having problems blowing through if you do. If you want a 2000 finish, skip the 1500 and go straight from 1000 to 2000. Most non-pro's can't tell the difference even from 2' away. The guide coat will also help a lot when it comes to hand sanding the edges at rubrail, sheerline, and waterline with 1000 wet, followed by the 1500 or 2000 depending on which way you go. It will also keep you from sanding any given area twice, it makes a huge difference especially for novices. Re-guide coat between grits. Polish with 3M Imperial Microfine on a foam pad, not a wool.
Finish with 3M Imperial Hand Glaze, applied and buffed off with terry towels by hand. It will come out looking better than a good shoot as long as you dont blow through. There will be no peel, and you can make the finish as nice as you want. It just takes time and hard work.
If you decide to re-shoot, whatever you do don't prep with 320 by hand. 320 is much too coarse and will show through the topcoat, especially when hand sanding. Use 400 dry on a DA. The random orbit really helps the grit scratches not show through. If you do blow through, but only in a couple of spots, you can just shoot those spots and blend them. Good luck!
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Old 07-01-2012, 18:44   #8
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Re: Polishing linear polyurethane imperfections?

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Originally Posted by cburger View Post
Linear polyurethanes and acrylic urethanes are two distincly different products. In your first post you indicated LPU as such I replyed accordingly. If what you have is truly an acrylic urethane this product can indeed be wet sanded and polised, then waxed. You need to insure that you have sprayed enough coats as you will be removing paint and if to thin it is possible to sand through to the primer or subcoat. In the hands of someone who knows how to sand and polish this type of paint it is possible to get a near perfect result. Speak to the manufacturer of the product to get additional info on their paint. Good luck.
Thanks again for your reply. I'm sorry if I misled you originally. I had always assumed that lpu meant the two part polyurethanes as opposed to the one part and that the difference in lpu's is acrylic vs. polyester (Awlgrip). (I notice that Interlux Data Sheet describes their Brightside one part polyurethane as linear. So, I'm beginning to be totally confused by terminology.) Anyway, if it's wrong that's the way everyone I know uses it, although not to forgive its misuse. I'm curious what your source of information about the word is. I'm beginning to think that linear is not a well defined term. I guess I should be more careful in using it. Again, thanks.
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Old 07-01-2012, 18:47   #9
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Re: Polishing linear polyurethane imperfections?

I have to ask and I'm sure you can figure it out but after a sand and buff will there be enough product left?

Are you better off just sanding with 240 and getting someone to re blow without the runs and orangepeel.

Another though, really, how bad is it?
After being in the water for a few mths do you reckon you can learn to live with it?
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Old 07-01-2012, 19:47   #10
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Re: Polishing linear polyurethane imperfections?

A really bad hanger can be sanded out with the least risk of blowing through by taping off around it with single layer of fineline with no overlaps in the tape. Then block it out with a hard block and 800 wet. Do this on all hangers before guide coating and sanding the rest. You'll find more hangers after you start sanding, the guide coat will make them obvious, but the worst ones can be hand blocked first. If you don't do this you may blow through in the area around the hangers before the hangers are sanded flat with the DA/1000 grit film. Never ever try to hand sand a topcoat, your hand is not flat. Even for radii and heavily contoured surfaces I use at least a foamy. Try hand sanding on a freshly guide coated surface and you'll instantly see what I mean.
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Old 08-01-2012, 16:00   #11
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Re: Polishing linear polyurethane imperfections?

Here is a picture of the bootstripe which shows both a run and the heavy orange peel. Is that a run that should be blocked? I understand the danger of sanding through the topcoat with too heavy a grit but I can't believe I have enough days in my life to sand it down starting with 1000 grit. So, I"m thinking of starting with 400 (or maybe 320 as that is the finest pad for a velcro DA sander which I can find here). Figuring I might have to sand it I put on 5 coats. I guess if I do go through I can spray touch up and polish that. Also attached is a picture of the (velcro attached) buffing pad I used on a test panel. I'm not sure of when to use foam pads, sheepskin, or this corduroy pad. Another question is when do I wet sand (not with my electric DA sander) and when do I dry sand. Also, since the polyurethane cures completely in several days what's the advantage of waiting longer than a week to do this? I really appreciate the help I've gotten here so far.
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Old 08-01-2012, 17:05   #12
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Re: Polishing linear polyurethane imperfections?

Quote:
Originally Posted by Epicurean View Post
Here is a picture of the bootstripe which shows both a run and the heavy orange peel. Is that a run that should be blocked? I understand the danger of sanding through the topcoat with too heavy a grit but I can't believe I have enough days in my life to sand it down starting with 1000 grit. So, I"m thinking of starting with 400 (or maybe 320 as that is the finest pad for a velcro DA sander which I can find here). Figuring I might have to sand it I put on 5 coats. I guess if I do go through I can spray touch up and polish that. Also attached is a picture of the (velcro attached) buffing pad I used on a test panel. I'm not sure of when to use foam pads, sheepskin, or this corduroy pad. Another question is when do I wet sand (not with my electric DA sander) and when do I dry sand. Also, since the polyurethane cures completely in several days what's the advantage of waiting longer than a week to do this? I really appreciate the help I've gotten here so far.
Ouch. Definitely a hanger which needs hand blocking, and any other like it. That's a lot of peel. With 5 coats, and it looks like they were very heavy ones, you may have enough material to do it though. All you can do is try and see. 320 or 400 is much too coarse for topcoat, it will take it right off in seconds on a DA. Won't leave enough material either, because then you'll have to work your way up through all the grits, and each one will remove too much material. I might start with 800 instead of 1000 on this one, but I doubt it. 1000 on a DA sands surprisingly quick, it's the wet hand sanding that will take forever. Maybe 1000 DA and then 800 wet followed by 1000 wet for the hand sanding. The guide coat will make it go much quicker too, you'll know exactly when you've sanded enough to remove all the peel and can then stop without overdoing it. It'll make it very clear whats been sanded and what hasn't, and will help you get close to the edges to reduce the amount of hand sanding without blowing through. Try some 1000 on a DA and you'll see it cuts almost as fast as say 600, it just dulls faster so you use more discs. Film discs require an adhesive DA pad, not velcro. The velcro would be much coarser than the grit otherwise. I'd guess I could 1000 grit DA your hull with 1/2 box of film discs and a 10 hr. day. The hand sanding, however, could take several days. While the poly does technically "cure" in a couple of days, you'll find in the MSDS that this is an "initial cure" which just means it's safe to touch or walk on. A full cure takes about 30 days. During this time the paint is bacoming harder and shrinking substantially. The only real factor for you here is gumminess, 1000 grit and higher hates to sand a gummy surface. It may feel totally hard and dry to you, but when you start sanding dry, if you keep getting little chunks of dust buildup on your disc, it's becuse you havent acheived enough cure yet. Ambient temp. and humidity play a big role in cure time. Sanding dry is much faster and less messy with a DA for obvious reasons, safer too. Otherwise you need to find a way to keep the surface wet while you DA sand, and not too wet or you will have problems with water slinging and self electrification. Thats why I prefer to get a decent cure and then dry sand with the DA. I wet sand with a block by hand, wet sanding is designed to remove the problems of gumminess as all the dust is washed away in the water. Try to polish out an area before you commit to too big a process, so you can see if you have enough material to polish it out without blowing through. Don't worry about how much material is left on the surface, as long as you dont make it so thin it becomes pertially translucent, a problem especially with whites and warm colors, you're fine. A thinner coating actually lasts longer than a thick one, only drawback is it's easier to scratch through. That's one reason pro painters blow it on as thin as possible while still acheiving flow. Hope my diatribe here helps.
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Old 08-01-2012, 17:12   #13
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Re: Polishing linear polyurethane imperfections?

Oh, and throw that corduroy pad away. Wool pad for polishing or buffing gelcoat. Never touch paint with anything but a foam pad.
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Old 08-01-2012, 18:00   #14
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Re: Polishing linear polyurethane imperfections?

OK I think I understand. We don't have adhesive backed pads here, as far as I can tell. I have another DA sander with a rubber pad that I contact cement discs onto. I could cut out 1000 grit discs from paper and glue them on. A little time consuming, but don't know if there's another option here.

Do I understand this correctly: DA sand 1000 then wet block rub (back and forth) with 800 grit then 1000 grit? How do I know when I've reached the end of each stage?
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Old 08-01-2012, 18:50   #15
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Re: Polishing linear polyurethane imperfections?

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OK I think I understand. We don't have adhesive backed pads here, as far as I can tell. I have another DA sander with a rubber pad that I contact cement discs onto. I could cut out 1000 grit discs from paper and glue them on. A little time consuming, but don't know if there's another option here.

Do I understand this correctly: DA sand 1000 then wet block rub (back and forth) with 800 grit then 1000 grit? How do I know when I've reached the end of each stage?

Yea, you got it, for the first step. Then do the same again with 1500. You cant contact cement paper to a foam pad though, it'd never be flat enough, the contact cement would be too lumpy. For me, I have a new quality porter cable DA with a fresh stock 5" pad that I only use for 800 git film discs and up. Sanding coarser grits and rough use can result in a sander which isn't quite "fine" enough for this use. A quality pneumatic random orbit is also a good choice. The slightest lump of goo or imperfection in the pad will ruin the job with swirlies. Buy a new pad for your DA and ONLY use it for this. They're fairly cheap. Film discs are super thin and really need that new pad. What kind of DA are you using?
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