What process do you use to get a good finish? Depends on who you ask what a good finish is. The bottom line is how much time do you want to spend on it?
Here is what we do:
Micro fill pretty much the whole airplane and sand away what isn’t smooth using West Systems epoxy. Yes, you could use EZ Poxy, if you enjoy sanding concrete. West Systems is way easier to sand and cures faster.
We use a LOT of power tools here. DA sanders with 36 grit sandpaper is the starting point. Let’s say you are doing the Canard first.
Use the DA with 36 grit to break the surface of the micro first. Then Lance likes the in line air sander... it’s about 14” long and 2” wide. Just like in the plans for the Long Eze, you sand with the sander length wise and on a 45 degree line.....in other words, not straight front to back but at an angle.
WHEN you get down to fiberglass STOP SANDING. Use your hand and find your low spots and circle them with a pencil. Do not use a marker here use a #2 pencil ....this is finish work. A marker stands a good chance of bleed thru. Speaking of bleeding, no bleeding on the project. Blood has proteins in it and will cause fish eye in the primer. If you do get blood on it, immediately clean it with acetone followed by wax and grease remover.
So now you have refilled low areas. Sand again with 36 grit until you get the contour which makes you happy.
We use a 6” roller or cheap bristle brush and give it one complete coat with PPG K38 primer. Let it tack off and repeat 2 more times. Spraying the primer on at this point is a bad idea. You will have pin holes in the primer and spraying the primer on will bridge the holes and not fill them. Use a roller.
This will be ready to sand in about 4 hours......yea....it cures quick. Hit it with a sanding block at least 12” long sanding in the direction the plans call for with 80 grit paper. Keep the block parallel in length to the span of the part and move the block 45 degrees to leading edge of your surface front to rear. Look at the plans on sanding and it show you the direction of sanding. Sometimes using a block 12” long is not practical so we use a 9” DA sander. The more you use and the longer your sanding block, the smoother and flatter your finished product will be. It’s a pain, literally. But hey, how smooth do you want it?
As soon as you get down to the fiberglass underneath, STOP SANDING! If you sand the glass you are getting into your structure and weakens your stuff.
Note: you WILL have pin holes; little areas which like tiny craters. You can find these by vacuuming the surface THEN blow it off with a air gun. It’s normal. These form usually from air bubbles in the primer and in the micro itself. Circle them with a #2 pencil so you can find them easily.
Get the roller and primer out again and put on 3 more coats. Use the end of your roller to put extra primer in the pin holes to fill them.
Once this cures hit it with 80 grit just to break the surface of your freshly cured primer. Then switch to 180 grit until you find glass again. STOP SANDING. Vacuum your surface, blow it off and go circle ALL of your pin holes.
Get some Glazing Compound....not Bondo, Glazing Compound is thinner than Bondo, and fill your pin holes and low areas with it. Glazing Compound sticks to primer.....not so much to fiberglass. It’s formulated for this. It cures in 15 minuets. Sand it off with 180. Use your block. You only want the holes filled, no extra material left over. Also Glazing Compound is not meant to be used for deep filling like micro. If it needs to be more than a mill or 2 thick, back up and sand to glass in the local area and fill with micro... start over.
Get your spray gun out and spray 3 coats of primer on. Leave it this way until you are ready to paint. Then block sand it with 180 just to break the surface, then use 320 grit wet sanding just to get past the 180 scratches. If you go back down to glass, congratulations, you get to spray more primer. You COULD use finer than 320 grit paper......if you don’t want the paint to stick. Seen it many times. 320 is as fine as you go with this.
So to recap:
Micro fill the big dips and sand with 36 grit.
Roll on 3 coats of K38.
Sand with 80 grit....90% of what you put on will be on the floor and 90 % of your defects will be gone.
Roll on 3 coats of K38.
Break the surface with 80 grit and finish sand with 180. 90% of what you put on will be on the floor and 90% of the defects you had before this primer coat will be gone.
Fill pin holes with Glazing Compound and sand.
Spray on 3 coats of K38.
Break the surface with 180 and finish wet sand with 320.
This is how we get airframes ready for paint.