Adhere to the Prime Directive -- "Put it on once, sand it off once."
Step 3: The Big Fill
The Big Fill is all about adhering to the Prime Directive, which is, "Put it on once, sand it off once." You'll find out in the next step that when you sand a part into contour, you sand down the micro until you hit the high spots. Then you stop. You don't sand any lower than that. For the prime directive to work, you must cover the entire part with one coat of dry micro that's thick enough to be thicker than the highest high spot. If you've done the prefill properly, then the coat for the big fill need only be somewhere between 1/8th to 1/4th-inch thick. But, apply more than you think you need. Let me say that again, apply more than you think you need! It's far better to have it on a little too thick and sand it all off once than it is to re-fill and re-sand the low spots.
The Big-Filling Process
1. If you haven't done so already, make sure to prep-sand any raw fiberglass and vacuum it thoroughly. (You do not need to prep-sand any of the micro that filled the low spots. But do vacuum thoroughly.) Again, you don't have to go hog wild on the prep-sanding. All you really need to do is scuff up the surface a bit to provide a good mechanical bond for the dry micro. You do not need to remove all the glossies. It certainly helps to do that, but it's not necessary to remove them all. I sandblasted the really large parts, like the wings and winglets. I hand-sanded the smaller parts with 36 grit sandpaper.
2. The plans say to apply an epoxy wipe to the surface prior to spreading the micro. An epoxy wipe is applying raw epoxy to the surface, then wiping off as much of it as you can. The idea is to let it cure to the tacky stage to help the micro stick to the surface and keep it from rolling up behind the trowel. Even though the plans say to do this, there's really no need. The dry micro has enough epoxy in it to adhere tenaciously to the fiberglass surface. Whether you do this or not is up to you. Some builders swear by the epoxy wipe. Some go a step farther and smear on a very, very wet micro wipe. I've tried it both ways. I don't notice any difference except that I save time by NOT doing the epoxy wipe. With proper spreading technique, the epoxy wipe becomes unnecessary.
3. Mix up a batch of dry micro. (See the previous page for how to do that.) Without delay, start scooping the dry micro out of the pail and get it spread onto the piece as soon as possible! You want to get it out of the bucket and spread out thin to avoid an exotherm. Left in the pail, that mass of micro will exotherm faster than you can say, "Nat Puffer is the Cozy designer and I will follow the plans exactly!" Ahem. Warning! NEVER set a pail of dry micro on any part of the plane for whatever reason. If you forget about it and it exotherms on you, it will melt a hole in your airplane!
4. Okay, with the public service announcements out of the way, it's time to spread the micro. I use a drywall trowel that has a 6-inch wide, spring steel blade. I hold the trowel at the top of the blade with both hands. Fingers on the top, thumbs on the bottom. I don't use the handle at all. I pull the trowel towards me with the blade held very flat to the surface. I apply lots of pressure. The low angle and the pressure on the blade are what thins out the micro evenly and what pushes out the air bubbles that inadvertently get mixed in with the micro. (Note: The 6-inch drywall trowel will not work on parts with compound curves. I use smaller putty knives and flexible squeegees for that.)
The technique is to plop the micro onto the center of the part and spread the micro from the center outward, following along the contour of the part, never against it. If you're doing a wing for example, you would plop the dry micro over the spar caps. Stand in front of the wing, take half of the micro, and pull it towards you to the leading edge. Then stand behind the wing and pull the remaining half toward the trailing edge. Once I've made the first pass, I make the adjacent pass with a slight overlap onto the previous pass. Don't be alarmed when ridges form off the edges of the trowel. You can knock the ridges down later. Remember, you're working against a time limit, so keep spreading the micro as rapidly as possible. Don't stop to answer the phone. Don't stop for the potty either. Realize that it will take several batches to cover a large piece completely. The WEST tends to kick off really fast when mixed in large quantities. Some builders talk about being able to mix up an entire gallon of WEST dry micro and get it spread onto a wing before the stuff kicks off. I guess I'm too slow and don't have that kind of speed! I go for smaller batches.
The most frustrating part is getting the micro to spread out evenly without it tearing, lifting up, or rolling up into a ball behind the trowel. The trick is to go slow enough to give the micro time to spread out from under the trowel and stick to the surface. You'll notice that the micro doesn't want to stick at first, but give it a few seconds. The epoxy within the dry micro will migrate outward and will wet out the surface. Once this happens, the micro will adhere and will spread out easier. (This is why some builders like the epoxy wipe.) Another trick is to use a heat gun. I start by gently warming the micro that I'm going to spread out. I then point the heat gun at the back of the trowel and gently heat the blade for a few seconds. When warmed, the blade softens the micro -- (makes it less viscous) -- and it will spread like butter! Use only GENTLE HEAT. You can tell when the blade is too hot. The micro starts smoking and it exotherms onto the blade! I can sometimes get away with holding the blade of the trowel in one hand while holding the heat gun in the other. I alternate between gently heating the micro and heating the back side of the blade. If the micro breaks up or rolls up behind the trowel, simply turn the trowel around and go back in the opposite direction. Rollups are caused by a number of things, including pulling the trowel too fast, attempting to spread the micro too thin, or because the micro wants to stick to the trowel. This is why I heat the back of the blade. It "melts" the dry micro slightly and allows the blade to slide more easily over the micro without the micro sticking to the blade.
5. Once the micro is spread out, now it's time to inspect it and try, TRY, TRY to fix imperfections. I say "try" because you can cause more trouble for yourself by trying to fix spots. Once it's adhered onto the surface, the micro doesn't like to be disturbed. If moved or spread again, it has a nasty habit of pulling off the surface and sticking to the trowel. You'd be better off just leaving it alone! But if you're like me, your technique is going to SUCK BIG TIME for the first hundred hours or so until you get the hang of it. So, you'll need to go back over the surface and fill some spots where the micro skipped, fill some areas that are too thin, (if it's too thick, don't worry about it, leave it be), or squeeze out air bubbles trapped underneath. To fix spots where the micro skipped or where it's too thin, just plop down a smaller blob of dry micro in the center of that area and spread it out as before. As you near the end of each re-spread, try to "feather in" the new micro into the surrounding micro by raising the trowel away from the surface just as you end the pass so the micro won't lift with the trowel. (A heated blade definitely helps here.) Air bubbles are caused when the trowel is moved too fast for the micro to adhere to the surface, but not fast enough for the micro to break from itself. An air bubble in the dry micro looks like an air bubble that's formed under the crust of bread dough. To fix it, just poke the micro with the corner of the blade to release the air, then push the micro back down onto the surface. I heat the blade, then pull across the surface just for good measure. After the first pass, you'll also have rows and rows of ridges that formed off the edges of the trowel. I knock these down by warming the blade and gently dragging the blade over the ridges with little to no pressure. I just let the weight of the blade do the work for me. In general, don't worry too much about what the surface looks like. All you care about is getting the micro onto the surface with a thickness higher than the highest high spot. True, the smoother the micro, the less work it is initially to sand off. But it's not a mandate. Remember, you can do more harm than good by excessively and unnecessarily tweaking the micro. Spread it, fix the major spots, then WALK AWAY! Leave it to cure (or leave it alone until it greens). WEST dry micro usually cures hard enough in 8 hours to begin the contour sanding. (An alternative is to use the cheese grating technique, as the contour sanding goes much easier when the micro is greening.)
Variations to Big Filling
Nick Ugolini provided a few variations that I haven't tried yet:
"When filling the wing, strakes, whatever, I like to prepare the glass (sand blast, or sand scratch) then squeegee on a very wet micro coat, and squeegee off as much as I can. (This is the epoxy wipe, but using wet micro instead of pure epoxy.) I let that set until it starts to green. Then I put on a very dry coat of micro (extra thick on low spots) to the entire surface. Pay NO attention to repairing bad spots. Consider this a scratch coat. The dry micro will not roll up much at all due to the first coat. Let cure for a number of hours until it is just hard enough to sand with 36 grit. If you catch it right it will be SOOOoooo easy to sand. Reminds me of using a cheese grader with a hard cheese. (This is the cheese grating technique.) It just rolled off the surface. The goal is to sand the surface flat, take off the high spots and lower the surface down to just above the finished surface. Blow the surface off. Then apply the final coat. This coat is slightly wet to allow it to flow into all the gaps, holes, imperfections of the 'scratch' coat. Pull the spreader down tight against the surface as you are not trying to do any filling or adding bulk but to just filling (repairing) the bad spots left over from step 2. When you get done, the wing will look almost finished. Let the filler cure for a few day and then you final contour sanding. This technique ensures you only have to fill one time. You get all the low spots and the entire wing has one cure for the epoxy so it reduces (eliminates) the hard spots. I completely finished both wings in 2 weeks. Almost all of that time was just sanding on the finished surface, as it only took one Saturday to do all the filling, and I started sanding on Monday. I did not have to go back and ADD any additional filler. Fill. Sand. Prime. Done!"