Posted by modenacart on 07/31/10 - 4:15 AM
#2
Yes, you want to primer after sanding. There will be millions of pinholes in the fiberglass and the primer will help seal it and help you see the ones it doesn't seal. Believe me, you won't see all the pinholes unless you prime. Also, I would use a board sander to at least sand the sides after you have primed. If you don't, you will have a lot more waviness in your finish. I didn't prime and board sand the first time and I was so unhappy with the finish I bought a board sander and did it again.
Posted by ritr on 08/01/10 - 10:40 AM
#5
sdespop wrote:
What do you use for a primer? Also, what is the difference between using waxless vs. wax added gelcoat?
Catalyzed polyester resin based products (Gelcoat is polyester based) cure in the ABSENCE of air, so some medium is needed to block the air from getting to it as it is curing. In a mold there is no air between the mold and the part so it cures. When you splash a part off of an existing part, there is no air between the existing part and the splashed mold you are making so it cures.
And Yes, PVA and Mold Release wax is ALWAYS needed on the surface you are going to spray onto. Don't skimp here or you'll be sorry. The more layers there are the better.
Sometimes when in need of a perfectly flat piece to tab into a hole I shoot directly onto a flat sheet of glass to get a perfect surface. Then I add the fiberglass / resin schedule on top of it the tacky gelcoat. Again on the gelcoat side of the part there is no air so all is well... But on the last polyester resin/fiberglass layer, there is air so an air barrier is needed for a good cure.
In all cases you have to prevent the air from getting to the polyester resin base. WAX IS USED TO DO THIS. When a small amount of wax is added to the catalyzed resin base the wax is squeezed to the surface and forms the air barrier you need. You must wash it off between layers in order for new layers to bond the prior layer. Napthaa will do the job. OR better yet use UNWAXED resin (gelcoat) and just pray it on when the prior layer is still tacky. This will give you a good chemical bond. Then the last layer will need the WAX added for it to cure properly.
NOTE: Surfacing Wax is NOT mold release wax. It commonly comes in little brown glass bottles or is already in the gelcoat when you buy it from your supplier. I buy UNWAXED Gelcoat along with enough "Surfacing Wax" to handle the amount of Gelcoat I just bought.
NOTE2: Polyester based resins are catalyzed by MEK-P. Again, gelcoat is a polyester based resin with bits of exotica added to make it look so nice. MEK-P is time, temperature, and light sensitive. So store it in a cool, dry, dark place when not using it. A refrigerator is perfect. Even with storage in a refrigerator it only lasts about a year. Much less if you store it in your hot garage in the sunlight.
Improper MIxing is the leading problem with gelcoat failure. Stir catalyzed resin without bringing up bubbles but stir it till it is mixed thoroughly.
And be sure to use a timer to count down how long you have before it kicks. Once it kicks, your done, If it kicked in the gun, some of the expensive guns may present you with a real problem. You can delay the kick by working cool, taking care with the catalyst to resin ratio, and keeping excess catalyzed resin/gelcoat in the widest bottom container you can find AND keep that container in ice, and switch out & clean guns early and often. Every Little bit helps.
Practice, Practice, Practice and it's a piece of cake.
Rit
Edited by ritr on 08/02/10 - 7:39 PM
Posted by modenacart on 08/01/10 - 4:48 PM
#7
If it kicks in the gun all is not lost. Mine kicked in my gun and I just used a screw driver to knock it out. Gel coat doesn't like to stick to metal.
When you are board sanding to prep for spraying, be sure to use guide paint to find the high and low spots. You will be glad you did. I didn't use it the first time and my sides turned out so wavy I had to board sand again, this time using guide paint. I just sanded one of the sides with 220 grit and the difference is amazing. If you don't know how to use guide paint, look up board sanding on youtube.
Posted by mayares on 08/01/10 - 6:44 PM
#8
Avoid the grinder, it make too many low spots that you will have to fix before painting. I used an air sander with 40 grit on my son's 13' that we sanded all the cracking jell coat off of. I used west epoxy on the parts that were down to the foam for filler, and 3M marine body filler for smoothing the rest out. For the holes that were mentioned earlier, a couple tubes of Bondo glazing compound fills them nicely and is really easy to sand. We even filled the recessed floor seams with it to smooth it out. I just painted another coat of Interlux Poly on his interior today. We did two coats of Interlux two part epoxy primer on the bottom and top of the hull. The results are amazing. For gel coat spray they do make a special gun for that.