I have had bad luck spreading on this particular tooling gelcoat so I just tried spraying it. It worked good but is very thin, it jumped off before I could finish what was left in the can. My question is about recoating. I have had bad luck with shrinkage after laying a second brush on coat. Should I lay a second coat and if so how long should I wait? Or should I just roll with what I have, which is still barely see-thru.
If polyester, always obey the minimum layer thickness, no matter how it is applied. This is usually between 500 and 800 microns. (sorry, always have difficulty translating that to mils)
The reason is that the thickness helps the gelcoat cure properly, so reaching a degree of cure after which the gelcoat withstands chemicals (styrene from the subsequent layers) and prevent shrinkage. During investigations on surface quality it always shows that degree of cure is important in reaching decent surface quality.
Thanks Herman. I called the manufacturer and he said my past shrinkage tribulations were probably due to trying to brush on a sprayable tooling gelcoat. The shrinkage, which looked like skin, was due to the thickness differential. It almost looked like a chemical reaction.
After the first layer I let it setup until it was firm but still a little bit tacky, then sprayed my second layer. It worked out great. I was just scared after my past experience.
I suppose I learned my lesson, there are 2 types of tooling gelcoat. Sprayable and brushable.
Definately. The spray version usually holds an amount of acetone, which evaporates when flying from gun to mould. When brushing this gelcoat, the acetone has difficulty to escape. You can try to catalyse less, to give the acetone more time to evaporate, but it is a bit of a gamble, and in general you do not want to allow the gelcoat to attack the mould release for too long.