Fiberglass mold- get a glass smooth surface?

[PDA1]

I've made a fiberglass mold and want to get the mold's surfaces to be smooth as glass.


Presently the fiberglass cloth weave texture shows through so that needs to filled in or built up.


Adding more coats of Polyester resin will take quite a while.


The mold will be used to cast parts from high density urethane foam so easy “release” of the casting is very important. (I already have PVA, Partall #2 and #10)


Color of the mold isn't that important.


What would you recommend for achieving a smooth surface of the mold?

[sailvayu]

I have used Duratec spray-able high build before works well for low volume molds. If you plan to pull more than a few parts you might want to do it right and use tooling gel
Duratec Polyester Surfacing Primer

[PDA1]

Originally Posted by sailvayu

I have used Duratec spray-able high build before works well for low volume molds. If you plan to pull more than a few parts you might want to do it right and use tooling gel
Duratec Polyester Surfacing Primer

How do you apply it?

Spray?

If so- what equipment are you using?

Brush?

[sailvayu]

Spray, I just used a cheap spray gun with a open tip. It is catalyzed so it helps to do a gel test first.

[Krogensailor]

You get a very high grade surface in a mold by starting out with a very high grade plug. I dont know any other way

[sailvayu]

That and lots of elbow grease!

[Zach]

I also use Duratec's grey surfacing primer, it buffs out beautifully.

I'd start out and board sand with 80-100-120 and 3M's dry guide coat, so you know what is flat, what is low, and what just needs help.

Mark the lows, and shoot two passes of Duratec into them first. When that has tacked, shoot a pass of 20 wet mils... Which is just about as much as will hold on to a vertical. If you are using a conventional cup gun or gravity gun, its going to take a lot of product due to the thickness and overspray. I use a gelcoat gun for the first shot to get a good build on the surface. You can use foam weenie rollers, but watch that you don't over work the surface, there is a point of no return that once the product begins to gel you start making non-skid texture. I suggest if you roll, do not do it in direct sunlight.

Once you have the surfacing coat on...

I use a national detroit 8 inch mud hog, with 320 grit and work the flat surfaces, same as you would an automotive paint job. Curved surfaces I've got a Hutchins Orbital long board that works well... Then what is left gets a 16 inch hand sanding board, 3M hard rubber sanding blocks, and the occasional paint stir stick in a bucket of water so it bends nice and easy. I try my best to avoid DA sanders, as they aren't big enough not to put swirls in the surface, and they fit into low spots much better than any of the other things listed.

I use a lot of red scotch brite and foam backed velcro sanding blocks to scuff out low spots before shooting the next coat on. I tend to shoot a heavy coat first, and block down flat until I start to see highs, spot in the lows until I'm 80% straight block it out and then shoot a full cover coat. Keep your pencil handy and don't "paint outside the lines."

I use Adtech's P77, for deep lows. On top of that I like and 3M's Dolphin Glaze as it fills the pores and feather edges. P77 feather edges better than most, 3M's Dolphin Glaze is one of the best. They don't shrink and sand about the same texture as gelcoat and Duratec which means you don't end up with high spots that are made of a harder product than the Duratec.

Shoot on your final coat, I normally go 10 mils with a gravity feed gun and focus on laying down the perfect coat with as little orange peel as possible. (Don't get the gun so close in the corners that the air flow wrinkles the product...)

When that has cured, wet sand it out to 800 grit, and power buff it with Aqua Buff 2 and you can see yourself in the surface. Normally this is a two-three pass deal. You have enough product on that you can sand through the imperfections, so the first time you buff it you'll see more stuff than you could feel.

I don't like shooting on much more than 10 mils wet for the final coat, namely you can make high spots that take forever to block out with 800 grit... but if you drop down to coarser grits, you are apt to burn back through. More elbow grease, flatter finished surface, and no "haze" from sanding scratches because you never put any in the surface...

The biggest thing to keep in mind is that there is a difference between a smooth surface and a fair surface. You can get something shiny with a bunch of waves and ripples in the surface, which means that you either apply a lot of horsepower and a lot of time with a fine grit and don't pick on any one spot... Or you spend a lot of time with 80-120 grit and don't pick on any one spot and spray a lot of filler covering scratches.

If you don't spot fill your lows, and continually shoot a full coverage coat on you have to sand the thickness of the product off the entire surface that is not low, in order to bring the low spots up to level. If you don't sand the entire thickness, of the depth of the low spot off the surface... You still have a low spot!

That is why aiming for 80% perfectly smooth and flat works well as you can fill the lows while working down the surrounding high spots. It ends up looking a bit like a patch work quilt, but with a flat piece of aluminum angle and a bright light you can mark out all the lows that look like the shape of texas and either putty them or roll another round of Duratec on them.

After the final coat of surfacer, the final sanding... You aren't trying to sand off anything other than the surface imperfections in the coating. The orange peel and the like. Once you buff it, you end up glass smooth and fair.

Cheers,

Zach

[PDA1]

Thanks for all the ideas. What I'm concerned about is the spray application of any Gel coat or Orange tooling gel coat.

My mold is approximately 22" x 44" and slightly curved (concave).

Please keep in mind that the mold will be filled with high density foam to "cast" the parts I need. Then the cast part will be glassed over. So, a perfectly flat, wave free mold isn't necessary. But, a smooth surface is extremely important as the cast parts must easily release from the mold without ruining or damaging them.

My compressor is only 1 HP.

It seems that most gel coats need to be spray applied using a 2mm tip.

What equipment would you suggest for easy spray application of orange tooling gel coat....or even just simple gel coat? (It would nice to get an inexpensive spray gun at Harbor Freight).

[Zach]

If you are making 2 parts that is a whole different ball of wax from making 200 parts.

If you specify it, you can buy brushing tooling gel coats. In europe quite a bit of gelcoat is brushed, rather than sprayed for whatever reason... So it isn't an unheard demand. It is thicker and more thixotropic than the sprayable stuff that is easy to source in the US. I prefer to roll on the thin stuff we have, but I don't lay up parts over it when I do... and the quality finish when waxed is lumpy.

1hp will spray gelcoat, but you will have to thin it enough to go through a gravity gun to do it. A gelcoat dump gun won't atomize with the CFM flow you've got.

But, if you are making 200... A GP100 dump gun is the best. Nearly instant cleanup, stone axe simple.

If you are making 2, the gnarliest under coating or primer gun you can find with a 4-5mm tip. It won't lay out as nice and will drip and spit... but if it kicks off in the pot you are out $20 from Harbor Freight.

I have a Dewalt D55146 that is 1.6hp 5cfm at 90PSI that I use when I don't want to fire up my 17cfm gas compressor. The Dewalt it impressive, it keeps up with my dump gun and binks on the pressure pot.

What are you making, and will the finished fiberglass part be bonded in to a boat with finish gelcoat or paint?

Only reason I'm asking is you could layup the fiberglass in the mold and then pour the foam into the part and save a lot of finish work, even if you have more pieces to put together to create the part... at least the molded side can be finished with no real labor.

A molded part can have a rebate around the perimeter to allow more fiberglass work and leave a smooth surface over the majority of the part. Making the rebate deliberately lower than the glass thickness means you can tab the part together trowel putty, and spot gelcoat the tabbed area and buff it out to bring it up to the same height as the finished surface. Lots and lots of time savings compared to fiberglassing foam and working up to a paint grade surface on every part!

A lot of folks use sheet wax which has a self adhesive back to make the rebate. Plop it in the mold, lay up the part... Where the wax was, your tabbing goes to assemble the rest of the component. Some cost associated, but your tooling can be modified easily, as can your layup schedule.

Cheers,

Zach

[PDA1]

Zach, thanks for the ideas again. A lot of what you say I don't fully understand.

From what I understand, Harbor Freight doesn't sell any spray guns with tip sizes over 1.7mm.

I'd be casting only about 4 parts. The finished parts will be painted but not with gel coat.

Lay up in the mold, for me, isn't something I'd want to do- too risky.

Brush on gel coat seems like a good idea.

I've read that you can use a Preval sprayer to spray gel coat. What are your thoughts about this?

How about painting the mold with something like a bath tub refinishing epoxy paint?

[Zach]

The gun I was talking about, harbor freight used to sell for applying rubberized undercoating on cars. Pretty sure they would shoot marbles...

Anyway:

For 4 foam pours with no fiberglass being laid up in the mold, I'd be inclined to go find some adhesive backed mylar window tint and slick it out as best as I could and stick it to the inside of the mold, taping the seams with mylar packing tape and let it go at that. You will have your work ahead of you sanding out the fiberglass over the foam anyway, and whatever you gain by smoother foam will probably be a wash time wise for 4 parts.

Might be the down and dirty method that requires no expensive materials, or any more sanding on the mold.

If the foam you are using doesn't get hot enough to bond to polyethylene sheet you could even use that... though the foam finish may be more agricultural than you are wanting.

Zach

[ausaviator]

Whatever you are making, you could probably get it made in China for 1/10th of the cost.

[PDA1]

Originally Posted by ausaviator

Whatever you are making, you could probably get it made in China for 1/10th of the cost.

Oh how I wish what you said were true.

What I'm making sells for around $3,000 not including shipping. I'll probably spend around $600.

[PDA1]

Originally Posted by Zach

The gun I was talking about, harbor freight used to sell for applying rubberized undercoating on cars. Pretty sure they would shoot marbles...

Anyway:

For 4 foam pours with no fiberglass being laid up in the mold, I'd be inclined to go find some adhesive backed mylar window tint and slick it out as best as I could and stick it to the inside of the mold, taping the seams with mylar packing tape and let it go at that. You will have your work ahead of you sanding out the fiberglass over the foam anyway, and whatever you gain by smoother foam will probably be a wash time wise for 4 parts.

Might be the down and dirty method that requires no expensive materials, or any more sanding on the mold.

If the foam you are using doesn't get hot enough to bond to polyethylene sheet you could even use that... though the foam finish may be more agricultural than you are wanting.

Zach

A little "texture" in the mold isn't too much of a concern so long as I can get the casted parts to release.

Oh well. I'll give your ideas some thought.

Thanks for the help.

[ausaviator]

So what are you manufacturing?