I'm on the middle of filling and smoothing the roof panel today, almost done, and that made me think I should write about a few things I've learned about body work on this project.
Disclaimer: I am certainly not a professional body tech. What follows are some thoughts on tools and techniques that have worked for me. Almost certainly there are better ways to do things
Materials.
I don't use the "bondo" products you can find in most chain auto stores. I read somewhere that the filler in those products is talc, which can absorb water and lead to rust. Also, they are a "one size fits all" kind of product for general body work, not always the best product for the specific need.
1 - Base coat - I use "All Metal", which has aluminum powder as its filler material. It is extremely tough (and a bit hard to sand). I usually do one coat of this, maybe a second in extreme areas.
2 - Top coat - I use "Icing", this is a somewhat self-leveling and easy sanding product with a very fine grain.
If I was doing the entire finishing job myself, I would then follow with high-build primer and block sanding with blocking powder, but what I do is get the panels to 95% or better using the above two products and I let the professionals at the body shop do the final 5%. I'm just not good enough to go all the way to paint-ready for the level of quality I'm shooting for on the Retro Wrangler build. Jamie at the body shop might spend another hour on the panel taking it the last 5% of the way, to me that's $45 well spent because he's so much better than I am at this.
Tools.
3 - 3x21 belt sander with 80-grit belt. Yes, I actually use this for body work, but only for taking town the first rough coat of All Metal. I use a very light touch and keep it moving so it doesn't cut too deep. It is very much a "blunt instrument", so it will end up making low spots I have to fill on the next coat, but it makes fast work of the first rough cut.
4 - Pneumatic long board - mine is from Harbor Freight, cost me $29 on sale. Sandpaper comes in rolls 2 3/4" wide. It doesn't take material off very fast, I actually don't use this as much as the next tool. I use this and the next tool to make sure I get flat surfaces (and smooth curves). If you had a choice between this tool and the next one, get the next one for sure.
5 - Long board - This one is marked with a star because it is one of the most important tools in my arsenal. This one takes PSA (pressure-sensitive adhesive) sandpaper in 2 3/4" x 16" size. I made this tool, but you can buy them inexpensively in any auto paint shop. It's a great tool because you can apply the right amount of pressure and stroke to get the job done and it won't creat dips in your sanding - you must use this tool if you're trying for a perfectly flat surface (which we have many of on our Jeeps!) If you did all your sanding with a small sanding block like #6, you could easily get waves and dips in the surface - the long board prevents that. Using the long board is how I got the flat parts of my fenders as flat as glass. Can't do body work without this tool, especially on a Jeep where there are so many flat surfaces. I typically use 80 grit in this one. Don't do body work without one!
6 - Rubber sanding block. I use this for detail work and sharp corners. Typically with 100-grit sandpaper.
7 - Random orbit sander. This is for finish sanding, with 150 grit. This is only to smooth the surface and remove scratches, not to remove any more material than that, otherwise you'll get waves and dips in the surface.
8 - Straightedges - I have marked these with a star also because they are so important, I check everything I do with these, even curves. If you can see light between the edge and the surface, you've got a low spot. These are used at the filler stage, later if you'd be doing final sanding you'd want to use a blocking powder to show the high and low spots. The lower left photo shows me checking the side of the roof with the long straightedge. Just above my thumb there's a 1/32" low spot, not sure if you can see it in the photo, but the photo shows how I use it. Also, I use it to check surfaces before I put any filler on - if there are any high spots in the base metal, I hammer them down with a body hammer before the filler goes on. Can't do much hammering once you put on filler, and if you've got a high spot in the metal once the filler is on you've got a problem. Also, you really want the metal to be close enough to correct so you at most use a 1/16" layer of filler, perhaps 1/8 in a small random dip somewhere, but if you're off by more than that you've got more metal work to do before going to the filler stage.
So the basic sequence is like this:
- Check surfaces with straightedges to make sure there are no high spots in the metal and everything is within 1/16" of correct.
- Rough sand the metal to give it enough tooth for the filler to bond well. I use around 50 grit and typically use sanding disks in my angle grider for this task.
- Base coat of All Metal filler; rough sand it with the belt sander and finer sanding with long board with 80 grit, checking with straightedge all the time.
- Repeated skim coats of Icing, mostly sanded with long board and a bit with sanding block for the detail; 80-grit. Usually I do 4-5 skim coats, very thin coats gives a lot of control about how much the surface gets built up.
- Final sanding with 150-grit in the random orbit.
One final tip - sandpaper loads up and clogs a lot when sanding body filler. I use a file cleaning card (a fine, stiff wire brush) to unclog the sandpaper. Really saves on sandpaper (lower right photo).
I hope this write-up helps. It works for me.