I'm ready to flip the boat and fair the hull before painting. There are 3 coats of epoxy and glass on the hull. The keel is not glassed. On the whole, things are fairly smooth, but there are small valleys, high points etc. I'll need to use a fairing compound and then sand. What is best for the job. My thought is epoxy with microballons and a little silica. What would be ideal proportions? Any ideas.
Dave
Just the microballons and epoxy to the consistency of thicker pancake batter. Leave out the silica it will make sanding more difficult and uneven. Spread it with a wide plastic spreader or taping knife. The wider the better. Like 12" or more. This will fill in the low spots. Let it set and use a long board to sand. The boards I found worked the best were either 24" X 4 inches 1/4" hardwood ply. I also have a couple of UHMV long boards I made out of sheet material. 24" X 4 inches X 3/8" thick and 18" X 4" X 1/4 thick. I purchase 4" X 100 foot rolls of tacky back in 80 grit and 120 grit sand paper and cut off what I need. I attached a couple large round drawer knobs to each end and work in all directions.
I go over all of it with the 80 grit and then re apply the filler in broad sweeping strokes held tight to the surface. The idea is to fill the low areas and only lightly knock down the high spots. This fills the low spots and then it is usually ready for a final sanding with 120 grit and ready for primer. I tint my high build primer coats different colors so I can see where the high spots and low spots lie and when I"ve cut thru a coat of primer. 2-3 coatrs of primer and I'm ready for paint. I wet sand the final coat with the shorter board and 150 grit wet/dry paper. Then wait a day or two for every thing to be completely stable and then paint.Â
On vertical or inclined surfaces some silica will be necessary to keep it from sagging. Just a touch is all you need to control viscosity.
Sand on one surface at a time, such as the starboard side of the bottom or the port side of the topsides, etc. Begin at one end with your long board and sand at 45 degrees (there abouts) to the centerline of the boat. Don't sand on any other angle, just back and forth at this same angle. Do this from one end of the boat until you get to the other end. The idea is to do two things. The first is to knock down the high spots, which the long board does naturally. The second is to scratch the surface in one common direction uniformly. This helps identifies the low spots. To this end (identifying low spots) go back along the same panel with the longboard, but use the opposite angle to the one you used on the first pass down the hull.
This is the standard fairing technique used by automotive body shops and professional fairing crews. Long board the length of the hull in one direction, then come back the other way (on the same panel) at a 90 degree bias. On curved surfaces is best to bisect the axis of the curve so you don't carve into the surface. An example of this would be to picture sanding a 55 gallon drum that's lying on it's side. If you sand along the axis of the curve, you'll make flat spots, but if you angle the sanding at 45 degrees of the axis, then you'll follow the curve and just knock down the high spots.
The cross hatch pattern that develops as a result of this technique makes spotting the low areas very easy.
Change your sand paper as soon as you feel it's not cutting or if it's "dragging". Your job will get done much faster with fresh, sharp sand paper. You can waste 5 strokes for every one if using worn out paper, so spare your elbows and change paper often.
Once you've made the "down and back" pass with the long board, you have areas that the board didn't scratch and other areas where it seems you really dug in quite a bit. The areas where there aren't any scratches are low spots that need to be filled and the other areas where you really dug in are high spots that need to be knocked down a touch.
Very lightly spray a coat of fast drying primer (a light dusting is all you need) over the panel and let dry. Sand the boat again, just like above. You've removed the high spots, unless you had some real ugliness and the low spots are really visible now (all the primer is gone except in the lows). Mark the lows and remove the primer with a solvent soaked rag, let dry, then fill. When dry, again with the long board, except you know the areas that need special attention (the lows). You may have to do this a few times to get all the lows filled and level with the surrounding surfaces.
A pro will make three up and back passes with the longboard before they're ready for primer, the novice would be lucky with 5 up and back passes. A pro will make at least one up and back pass after the first few coats of primer have dried to insure the surface is fair. It's not necessary if you are brushing a paint job, but if you want a smooth, nearly flawless paint job you can smooth the primer.
Machine sanding will invariably remove way too much material, way faster then you can possibly ever think about controlling. A DA or other orbital sander is useless when it comes to fairing a boat (or any surface). These tools are for bulk removal or smoothing, not fairing. There are power longboards, typically called inline sanders. These are great if you have them, but they also remove material much faster then a novice can be expected to control. I've made power longboards by attaching jitterbugs and 1/3 sheet sanders to a length of plywood. These can save a lot of elbow grease, but I've warned you about the speed at which they will remove material, especially really soft fairing filler or primer.