I recently built three 14 ft race stand up paddleboards in carbon fiber. They were super light after lamination, but gained too much weight after adding the sanding coat and exterior paint.
I had one old windsurf builder tell me to do the following:
“get airocel and mix it to the consistancy of vasoline. Then
squeege it on before you hot coat. Get it on thin as it does not sand
well and pull from two directions to get those pesky little holes
filled. Then hot coat over this. After you sand Red Cap it at least
once of not twice, then sand the Red Cap back and paint”
Anybody know what Red Cap is?
I heard second hand, some guys squeegee microlight filler and epoxy over the lamination layer, then paint. Does that really seal the pin holes? Is the paint being used to seal pin holes too?
I was recently chatting with a builder of hollow carbon race SUPs at a local race. He said he knew the secret to doing really thin epoxy hot coats, but would not tell his secret. I was shocked by his attitude. I’m just a kook building boards for me the wife in the garage.
I want my next boards to be light like they were after lamination, or close to it.
If you have pin holes in the lamination it would help to do a cheater coat after the lamination has set up some. Before painting there are numerous fillers that sand easily such as 3M Marine Putty (p/n 051131-05962). When hotcoating with epoxy you can only go so thin because there has to be enough there to flow out. As for the hollow SUP guy he’s probably molding so he get’s his exterior finish that way. As far as Red Cap could that be the filler that the car guys use. Its is similar to the white marine putty.
I’ve built one carbon fiber surfboard with 6oz carbon fiber and I had the same weight issues with the hot coat. Problem is that since carbon is light, 6oz carbon fiber cloth is in fact a lot thicker and with larger weaves than 6oz glass. Thereforre, in order to get a smooth surface you need more hot coat resin and looses the weight gains from using carbon.
If I were to do it again, I think I’ll use several layers of smaller carbon cloth (2oz or 3 oz) and then one layer of 4oz glass that would be sacrificed when sanding.
Im definately no expert but I have thinned epoxy for filling the weave on a small RC boat I was building and really didnt want any extra epoxy weight. I warmed both parts of the epoxy by placing the bottles in a warm water bath - this alone makes it flow much easier. Once mixed I thinned the epoxy with denatured alcohol - It almost ran like water. From what I have heard the use of DNA will effect the epoxy strength – I was just trying to fill the weave. Again - this was a small RC boat that will not see anywhere near the abuse a surfboard will.
I have no experience with this process (yet) and it requires more $$ in hardware and consumable materials – but I do remember seeing something about combosite construction methods where they achieve almost a final finish utilizing vacuum bagging and a smooth release film / peel ply during the initial layup. As the vacuum is pulled the film creates a smooth surface layer and the weave is saturated, filled and forced flat/smooth by the film. Once set they remove the bag and film and sand out any slight imperfections - practically a finished product out of the bag. Im pretty sure that this process also reduces / minimzes air bubbles and pin holes.
Sometimes they used a perforated peel ply that allowed excess epoxy to be drawn away from the laminate during cure – reducing final laminate weight and improving the cloth/epoxy ratio and strength. Looked like the perforated peel ply left small nubs where the excess epoxy was drawn off that required sanding – but the weave is filled, requiring less filling/hotcoat/bondo, etc prior to paint or clear coat.
FYI - I have found that the good two part epoxy fairing compounds / fillers are expensive but they are relatively lightweight and leave very few pinholes. The cheap bondos are light but sometimes leave lots of pinholes that require additional fairing / sanding treatments prior to painting.
Dont know if this was any help - Im sure that the more knowledgeable guys here will offer better tried and true suggestions for you.