Building a Sea Chest

[bly]

Originally Posted by Blue_Heron

I've brushed gelcoat and I've sprayed it. I've gotten better results spraying it. If you have a compressor, spray is the way to go. I use a $16 gravity feed gun from Harbor Freight and thin the gelcoat 20% with acetone. Thin the gelcoat before you add the catalyst and use 2% catalyst based on the pre-thinned volume. Make sure you have at least 15 mils thickness of gelcoat.

For mold release, wax is good, PVA (Partall Film #10 if I remember right) is better for this application. PVA must be sprayed. I use the Harbor Freight touch up gun @ 60psi for PVA.
Dave

Your finish will be fine for a sea chest with pva. Alot of water is all you need for gun clean up. I also use water to remove my parts. from a mold. A wooden wedge to open a void enough to get water into the finished part will float it out of there and unstick the PVA back into a liquid. Lots of water for PVA clean up. No acetone or chemicals . Just helping a little to blue herons advice for a first timer.

[strick]

What are the dimensions of the box? The smaller the part the less layers needed to keep it strong. This does not look like a very big project and I would not invest in a cup gun unless you are planning on doing lots more fiberglass work in the future. You should be able to brush on the gel coat if you do it carefully. I would do 2 layers of 1.5 oz matt and 2 layers 1708 and be done with it. I don't know why you would need to core such a small box? Coreing the bottom may provide a flatter surface for your plumbing?.... but I don't think it is necessary. someone help me here......

strick

[mbmcdermott18]

Thanks for the info guys. I was going to core the bottom so I could run screws into the bottom to hold the pumps down.

the box will be about 21" wide flange-flange with the bottom of the box being aprox 17 3/4" the box will be aprox 10" wide flange-flange.

What is the difference between pva and wax? I have read of people using both and I have read of people using one or the other. Is one better than the other?

[mbmcdermott18]

Hey Blue Heron I assume since you are thinning the gel coat you are using a tip around the 1.5 size? Have you had any luck spraying unthinned gel coat out of a larger tip gravity gun like 2.3 size? I have a few other mold projects I would like to tackle if I can get this one to come out right so the money on the gun wouldn't be a loss but if a $30 gun works as good as the $120 cup gun I would rather save the extra cash.

I plan on ordering my supplies from us composites seems like they have good prices. I am going to use vinylester for the layup if that matters.

[Blue_Heron]

Originally Posted by mbmcdermott18

the box will be about 21" wide flange-flange with the bottom of the box being aprox 17 3/4" the box will be aprox 10" wide flange-flange.

What is the difference between pva and wax? I have read of people using both and I have read of people using one or the other. Is one better than the other?

strick is right. If your box isn't any bigger than that, you don't need to core it. strick has more experience than me on gelcoat application and layup. I've never been able to brush gelcoat without streaks showing up in the finished part, so I spray it. I would go with his layup recommendation too.

PVA is Polyvinyl Alcohol. It's basically a water soluble plastic. You spray it on your mold and it forms a film that looks like green saran wrap. Here's a link to a better description:

http://www.bottompaintstore.com/part...t-p-10321.html

They carry it at US Composites.

Dave

[mbmcdermott18]

http://www.harborfreight.com/air-too...gun-66103.html

Has anyone used the above gun for spraying unthinned gel coat? It comes with a 3/16" tip which is what the cup gun uses for gel coat.

[strick]

Originally Posted by mbmcdermott18

What is the difference between pva and wax? I have read of people using both and I have read of people using one or the other. Is one better than the other?

You are gonna want to wax and buff the mold several times and then spray it several times with PVA This is the best way to assure that the part does not stick to the mold.

I have not used the gun that you posted in the link but it looks like a good set up. I do have a cup gun for spraying gel coat and they are a little tricky to use. Because the cup is mounted horizontally on the gun the gun must be pointed up and the trigger pressed before you lower the gun or material will drip out the tip and make a mess. They let out a LOT of material. The gun that you posted looks much like a conventional spray gun and should be easier to use.

Your biggest challenge in this project will be a nice flat surface to mount your plumbing to so it will seal nice and even....in hind sight coreing it may not be a bad idea. I like Core Cell but on such a small project properly sealed plywood might be more economical.

strick

[mbmcdermott18]

Strick, you have me thinking about the mounting surface now. The two raw water supply thru hulls and the pickup for the washdown pump will primarily be sealed on the finished surface (inside the chest). So I think those would be ok.

I am however concerned about the seacocks which will be sealed on the outside of the box. From looking at the unfinished side of other parts produced in a mold it seems like it would seal fine after the application of some 5200?

Thanks to everyone for the tips.

[mbmcdermott18]

Mold is almost ready to go. My glass supplies should have been shipped from US composites today and I will probably begin glassing next wednesday or so.

If anything looks out of place or anyone has any tips let me know this is my first go at this type project.

[Blue_Heron]

Originally Posted by mbmcdermott18

If anything looks out of place or anyone has any tips let me know...

Your mold doesn't have any taper. You will have to destroy the mold to free it from your part. Male molds that are intended to be re-used always taper from the opening to the inside. In other words, the closed end is smaller than the open end so it will slide out easily once you break the bond with the new part. I'm not saying it won't work, just that you need to think about how you're going to get the mold out without damaging your part.

Second thing is to make sure you seal your MDF with something to make it waterproof before you go any further. PVA is water based, and the MDF will soak up the water and the PVA won't want to dry. It will also probably raise the grain and make the surface rough.

I use clear lacquer on my molds, but I use masonite as the mold material, and that's smoother than MDF. Strick uses Duratech primer. I'm sure there are other coatings that would work, but Duratech is proven for MDF mold surfaces.

Your work looks good, though. Nice tight joints and good fillets and outside corner radius.

Dave