If it was a Clark blank, your resin kicked off app. 45 minutes after mixing, and the pinbubbles still appear, it’s your technique in rail saturation. Apply more resin, don’t be stingy, squeegee the rails at least twice after initial saturation.
The rails are getting bubble from over working the glass to lay down. Fiberglass is like a sponge, the more you fuss with it and pull the squeegee around the more chances you will wring out the resin from the cloth. This is very easily and most often done on the rails, nose, and tail corners, you hardly ever see bubbles in the middle of the deck because you drag the resin around saturating your cloth, then you plow back and forth making it smooth and flat with a low angle of the squeegee. When you get to the rails you’re more than likely using a too high of an angle as you turn the rail and your pulling too much resin out of the cloth. You want to use the same low angle and just help the saturated cloth lay down on the foam. Yes, you want to make sure its laminated properly but, if you over bear on it you’ll pull all the resin out and flatten the cloth. Once this happens the cloth tries to regain its original shape and starts sucking air and you get millions of pinholes. As for the weight of boat cloth? it could be any where from 6 oz to 36 or more ounce? Does it look like a burlap sack, or is it thicker or thinner?
If it’s too heavy you won’t be able to turn your rails because it’s too stiff/ thick, that too might be causing your million bubble sucking air pinhole lamination problem.
I think Lee and Resinhead are both right, in addition you might be seeing a CHEMICAL issue coming through. Most all fiberglass gets a series of washings. Depending upon their end use, the washings will vary greatly. It turns out some of the most demanding washings are needed in the production of surfboard fabrics. Clarity is the biggest issue, which allows logos to show through, airbrushes to pop, and boards to look killer in the surfshops. Chemicals such as Silane are bonded to the cloth to get the refractive index just right to maximize clarity in the most effective part of the visible spectrum of light. Other chem washes promote binding of the resin molecules to the cloth, and emulsions to allow the resin to flow quickly. I have been told that there are perhaps hundreds of different washes used. You will notice very different behaviors in laminating with different brands of the “same” cloth, and even within a single brand but a different lot. To get to your problems. I am guessing the boat cloth has a very diffierent wash on it. Much less demanding in the clarity department and probably formulated to bind with vinylester or iso- resins rather than the ultra/overly refined surfboard polyester resin. I can almost bet that the wash is not letting the resin stay soaked on the glass filaments so when the glass gets bent around a rail (vertical drainage potential) and physically worked, it makes sense that the dry-cloth/drain-out/bubble-filled texture is showing up. May I suggest that you change to surfboard cloth (weaker/clearer) OR change to boat resin (stronger/dirtier). Ask for vinylester resin since it is so strong, bonds to the boat cloth and is still not too pricey. Obviously, since you bought boat cloth, you don’t care about the cosmetics that much. You would be surprised how much stronger your boards will be if you build them like boats…