Earlier I mentioned that I was going to use Bondo for the fillets and edge smoothing, and I got a lot of info to the contrary. well the bottom is covered in fiberglass and resin. The question is what do you use to fill the rest of the weave where the resin shrunk in, and the sides where the drips occur? Is there a trowel on product that you use before painting, or is it another coat of epoxy and a {healthy} sanding, Thanks Marc
To fill the weave, most people use another coat of epoxy, often thickened with wood flour (use the purchased stuff - what's in your sander is probably too coarse), or micro-bubbles of some sort. Autobody filler is another option but it won't stick to epoxy well and can absorb water.
There's nothing "healthy" about sandingÂ
 What I've done with the sags and drips is to attack them a bit more agressively using one of a surform shaper, an auto body rasp, or my new favourite - my wood rasp. Just be careful to not cut through the actual fabric if you can avoid it. You'll save 90% of your sanding time if you go after the big mistakes that way. Since these methods can leave score lines in the surface, I follow up with a skim coat of resin and then just sand enough to give it a tooth for paint.
Remember - it's a boat, not a piano.
Thanks I'll give it a light sanding then recoat it with the epoxy, and move on.
What I don't understand is that bondo makes a lot of different poducts. Including a bondo version with fiberglass in it and i know for a fact that it is 100% waterproof as it is a marine product made for boats, not automobiles. Why could'nt you use that to use a filler or smoothing agent before painting?
Bondo (3M products) offers a number of products, but all are polyester or acrylic based, making them either un desirable on epoxy or less then waterproof. It's parent company offers products under other brand names that are far superior.
The products you might be thinking off are not the fillers you want. These might include Bondo-Glass, Bondo-hair, etc. These are polyester resin based products and not fully waterproof. They are designed specifically for use with production built boats or for automotive repairs. Interestingly enough, good body shops and custom body shops have switched to epoxy based adhesives and fillers for repairs and dropped the polyester based stuff. They've done so for the same reasons the marine industry has.
Using these pre-mixed filler/resin products isn't the most economical way of providing a filler mixture for your boat. Most of the resin formulators offer pre-mixed stuff now. You pay a huge premium for this, somewhere on the order of $100 per gallon of filler/resin, which is over twice the wholesale rate. I'm paying about $40 a gallon for resin and make or purchase fillers, which of course requires I mix my own. If you mix your own, you'll be able to have custom mixes to suit the environment and need of the particular day or job, as well as the cost.
In the end it's your call, but if you need say 5 gallons of epoxy for a boat project with two gallons of assorted fillers, you can control costs a good bit with mix it a yourself approach.
To fill the weave on a freshly glassed hull, don't just use resin. This is wasteful, particularly on low viscosity resins (like Raka). Use a light weight filler, such as Q-cells or micro balloons. Both of these will need some silica to "stiffen" the mixture and prevent sags or runs, but it's all part of the process. A light skim coat with a wide blade knife will do. It's better to make multiple coats of this light weight filler mixture, then one thick, try to get it all in one shot application.
Fill the weave as best as you can, knock it down with a long board, mark your lows, fill the lows, then pull out the long board again.
Mike you can use what ever you want. We are just giving you the benfit of our experience as most of us have at one time used bondo as a filler. The long term results were less than desireable.Â
I will second what Paul said. Late in my boat's 'glassing, I discovered that if I mixed my epoxy with "colloidal silica" to make it about the thickness of mustard (mmm), and spread it on with a rubber or plastic applicator, it resulted in a smooth finish that required very little sanding, just what was required to prepare it for paint. The filler resulted in glass that was as smooth as... well, glass.
