hull side sanding progression suggestions after a bout with 40 grit

[theller]

I had to get really aggressive with removing an old coating from hull sides and deck. Like, 40 grit abranet aggressive. It's time to deal with the expected aftermath. Deep 40 grit scratching.

You might think: "Why the hell did you use 40"... we are 50 hours of sanding into this thing, with 40 grit. It just wasn't happening with 60/80/120. It all had to come off - it was pin-holed, everywhere. Easy on me folks.

Couple options I'm considering.

1. Go super high build on the primer. Sand to my hearts content until I clear the scratches.

2. Get at it again with 80 until I get through the scratches. Afraid I'm going to get real wavy with the surface finish because I'll be hunting the bottom of scratches.

3. Mix putty until my eyes start bleeding and skim coat the whole thing. rough that with 80, then follow up with heavy build primer.


My gut tells me go with a high build primer. Anyone got any practical experience with something this drastic?

Any guidance on how to play it safe? Maybe a small section first, count how many coats I need until I see the scratches fill?

[Uncle Bob]

Originally Posted by theller

I had to get really aggressive with removing an old coating from hull sides and deck. Like, 40 grit abranet aggressive. It's time to deal with the expected aftermath. Deep 40 grit scratching.

You might think: "Why the hell did you use 40"... we are 50 hours of sanding into this thing, with 40 grit. It just wasn't happening with 60/80/120. It all had to come off - it was pin-holed, everywhere. Easy on me folks.

Couple options I'm considering.

1. Go super high build on the primer. Sand to my hearts content until I clear the scratches.

2. Get at it again with 80 until I get through the scratches. Afraid I'm going to get real wavy with the surface finish because I'll be hunting the bottom of scratches.

3. Mix putty until my eyes start bleeding and skim coat the whole thing. rough that with 80, then follow up with heavy build primer.


My gut tells me go with a high build primer. Anyone got any practical experience with something this drastic?

Any guidance on how to play it safe? Maybe a small section first, count how many coats I need until I see the scratches fill?

Hi, my go to would be the high build primer and sanded with long boards to eliminate the inevitable waves if you don't. I think I would start with a finer paper, more work but better finish prospects.

[Fore and Aft]

Theller I did the exact thing to a fibreglass dinghy I am restoring. All I did afterwards was spray a heavy coat of epoxy undercoat then spatula it into the pinholes while the paint was wet and the 40 grit sanding swirls just disappeared in the paint. I thought I might have to undercoat again but I have just finished sanding with 240 grit and am happy with the finish. I still did a couple of spot fills.
Cheers

[theller]

Originally Posted by Fore and Aft

Theller I did the exact thing to a fibreglass dinghy I am restoring. All I did afterwards was spray a heavy coat of epoxy undercoat then spatula it into the pinholes while the paint was wet and the 40 grit sanding swirls just disappeared in the paint. I thought I might have to undercoat again but I have just finished sanding with 240 grit and am happy with the finish. I still did a couple of spot fills.
Cheers

Mate, that's great feedback! Fortunately, the pinholes were all in the topcoat that I removed, so I'm just left with sanding scratches to fill.

[Fore and Aft]

Theller I just finished spraying the dinghy and this is what the finish looks like going straight from 40 grit to 240 grit. Using Northane Shipshape epoxy primer then top coating with Norglass linear two pack linear polyurethane. The colour is mongrel red as I was short 250mls, of paint so I mixed some Northane orange with the red.
Overall, I could have spent more time sanding and probably spraying another undercoat, but I am happy enough.
Cheers

[phorvati]

Originally Posted by theller

I had to get really aggressive with removing an old coating from hull sides and deck. Like, 40 grit abranet aggressive. It's time to deal with the expected aftermath. Deep 40 grit scratching.

You might think: "Why the hell did you use 40"... we are 50 hours of sanding into this thing, with 40 grit. It just wasn't happening with 60/80/120. It all had to come off - it was pin-holed, everywhere. Easy on me folks.

Couple options I'm considering.

1. Go super high build on the primer. Sand to my hearts content until I clear the scratches.

2. Get at it again with 80 until I get through the scratches. Afraid I'm going to get real wavy with the surface finish because I'll be hunting the bottom of scratches.

3. Mix putty until my eyes start bleeding and skim coat the whole thing. rough that with 80, then follow up with heavy build primer.


My gut tells me go with a high build primer. Anyone got any practical experience with something this drastic?

Any guidance on how to play it safe? Maybe a small section first, count how many coats I need until I see the scratches fill?

You really can't cut corners on prep. You can but the finish will look exactly as good as your effort. You need to spray guide coat and work your way up. Guide coat is a light spray paint of whatever color you like that sits in groves of 40grit so that when you go to 80 grit you have a visual confirmation that you have leveled all the 40 grit groves. Its a PITA but after 80 you would go to 150 or so, then 220. All with guide coat in between. I've lived to tell the finish. Left some notes and pics along the way.

https://www.cruisersforum.com/forums...ip-102936.html

[Celestialsailor]

I did a 34ft. sailboat back in the 90's. From the factory, it was a lumpy potato. I used a D/A with 40-60-80, then used a lacquer primer which sands easily. This showed me my high spots. It took me weeks until the sanding of the lacquer looked consistent. then I used Awlgrip 545 high build primer. (Hint)...it comes in white and gray. My LPU color was dark blue, so I used gray. I also used a home made sanding board and learned how to sand at 30 degree angles, one way, then the other. After a couple of more spray coats, I was able to finally able to end up with 320 wet sanding...Good luck.

[Rumrace]

I wouldn’t use 40grit on anything I liked. 80 would be awful.
Water sanding is more effective than corse papers. Put a powered sander behind it you have sand embedded in the laminate.
Plastic melts when you sand it clogs the paper burns surface repeat.
In every paint there is some sort of dryer. In the case of gel coat it can’t cure in oxygen. So topical gel coat has a drying wax where as gel coat used in a mould has non. The best paints made are water clean up ( sort of) but never never never use water as a thinner. You will loose adhesion.
Your boat wants some wet 320.
I love the blue oxide primer from DuPont. Seems dumb to use rust proof primer on glass. It shoots nice levels and holds paint like a high security lock.
You can use clear with a tint as a primer also. Sand your second coat of primer. Gel coat and epoxy don’t sand the same. 800 polisher makes seams where repair meet old gel coat reduced. Again wet.
Base clear paints from DuPont are my pick for tough paint. I also like auto air out of Cali who buy pigment from DuPont. They are the original candy paint guys ( ink in clear). I use a drop gun usually a 2 bottle mix. A single bottle will put a coat on a Corvette.with some waste left over. A donzi 18 sucks up two bottles a coat. You need 3 coats of everything minimal

[pcmm]

If you've used 40grit you're not at a stage where you can just go to high-build primer. the gouges from 40grit will need proper fairing compound first to do some fill in before you get to the high-build. You need to repair the damage first.

The only time I use something that aggressive is when I'm creating tapers in glass to start a repair and want to remove a lot of material fast!

[Rumrace]

Yup. 40 is fubar.
I’d never remove the old gel coat. It’s the only thing partially water proof. Paint sticks to itself better than any thing else. I replaced the transom on my little donzi so where I cut into the old 80’s gel I sealed with epoxy.
You’ll need a barrier coat, and polyester will not work. It fuses with the glass mechanically once and super hot. You need to use the Vinyl alternative which are waterproof. Youll need fillers too. Micro balloons will help the epoxy make mechanical connections.

[BigBeakie]

Jotun Penguard HB will certainly fill 40 grit scratches. One coat can be sprayed up to 0.25mm DFT with no worries. It sands well to 320 and topcoat can go directly over it.

We used powdered builders chalk ( for re-filling builders string line) for guidecoats and just used a big car wash sponge dipped in it to spread it. Works just as well as the expensive guidecoat powder at 5% of the cost. Don't use the waterproof black chalk from Stanley though, it has something in it that feels greasy and stains the HB.

Sand the guidecoated HB with 80grit longboard, circle any lows with a pencil and fill spot with more HB, then guidecoated 120grit with torture board, then you should be fair. Time to then smooth sand with 320 on the random orbital/DA sander.

[theller]

Originally Posted by Rumrace

Yup. 40 is fubar.
I’d never remove the old gel coat. It’s the only thing partially water proof. Paint sticks to itself better than any thing else. I replaced the transom on my little donzi so where I cut into the old 80’s gel I sealed with epoxy.
You’ll need a barrier coat, and polyester will not work. It fuses with the glass mechanically once and super hot. You need to use the Vinyl alternative which are waterproof. Youll need fillers too. Micro balloons will help the epoxy make mechanical connections.

it's funny.. I heard a lot of advice in the yard "stick to 120" or 220. After a couple hours of mucking about with it, I talked to one of the pro's in the yard. After looking at the pin-holed paint on the boat, recognizing that it needed to be completely removed, he said...

"your friends don't want you to ever leave the yard"

...then told me to grind it off with 40 and build it back up.

This job is a full removal of a very hard, pinholed paint, epoxy primer to two part epoxy finish build up.

[Celestialsailor]

The whole idea behind high build epoxy is to add thickness like a fairing compound does, without introducing another irregular surface. I agree with Theller..."then told me to grind it off with 40 and build it back up".
But here is the thing about sailing forums like this...a lot of advice but no direct experience. Aside from fairing a couple of steel hulls I built (totally different method of faring), I have fared 2 f/g hulls and I can tell you it depends on the severity of what you're working with. My Olympian 34 had an irregular hull surface from the mold. Some dips and waves were a 1/16" and better. I knew if I put LPU over that, it would enhance the irregularities. In other words...look like $h!t. So I fanned the surface with gray lacquer and board sanded to highlight the high points and started with 40 from there. No body is saying you use 40 then paint. A statement like that points out the inexperience that goes on here. But as you remove more and more of the high spots, by lacquer fanning and board sanding, then you can begin to refine the grit of the abrasive. Then use a high build like 545 which is formulated to sand easily...sorry but you won't "clog the sand paper or embed sand into the laminates". At the 545 point you are starting to use 120. You will still be sanding through to the hull. Eventually you'll get to 220 grit and finish with a 320 wet sand. Then you can top coat.

[BigBeakie]

Firstly, 545 is not a high build primer, it is a finish primer. By the time you would put on a finish primer, your hull is already fair, so.....
Secondly, why would you be "still sanding down to the hull" if the boat is fair?

I think the OP may be confused by such suggestions.

[Fore and Aft]

Theller, the way I see it are you going sailing or are you creating a work of art? Your yacht is 46 year's old so for me l would forget long boarding fairing etc. Spray the undercoat, slap on some guidecoat, sand that of and then topcoat. No ones mentioned that no matter how good of finish you have on the undercoat if the topcoat is not applied properly then all that effort has been wasted. Runs and orange peel in the topcoat can't be hidden. Same with poor preparation in the undercoat. I once had a dispute with an owner about how long I was taking to prep his yacht for topcoat. The owner sanded one side and I finished sanding my side. When I sprayed the topcoat the owners side looked like a golf ball and my side looked like the paint on your car.
Theller you haven't mentioned what sort of finish you want to achieve. On my old man's trawler we used to heavily sand the topsides then roller on the cheapest white oil based paint we could find.
Cheers