What ever tools you use the trick is to not add the last "skim coat' till you KNOW that it is all you need. Don't try to block out that first coat, just use it as a base for the LAST skim coat.
I was taught this procedure after doing bodywork for a number of years and it really works well:
Just apply a nice coat of filler (what ever brand, whatever style, we will put that aside right now). Cut that coat NOT to make it perfect, but to get the basic shape and filling you need as a base for the skim coat. You can cut it with 36 40 or 80 depending on how big the area you are working is. In other words, if you can cut it fast with only 80 then do it. But I would say that this would be limited to an application that is no larger than about 8 inches.
If you happen to have a few high spots, see if you can tap them down.
If you have a few low spots add a bit more filler to ONLY those spots.
Re-cut these last low spots you have just filled with the same grit you have been using (most likely 36).
If you now have a surface that ONE skim coat will fill, then apply it. If you don't work with it a bit more, but NEVER add a little here or there and think you will finish it without a skim coat.
If you have a surface that is very close with only a few VERY MINOR low spots like poor feathering onto the metal, poor transitions from one application of filler to another, or from the metal that is "poking" up here and there you can do the LAST skim coat.
This skim coat is very important, you want it to extend over the COMPLETE area, this is well past the damage you have been working. Maybe as much as 3 inches past the plastic that you have applied to "rough" it out.
This skim coat can be regular filler or a polyester glaze like "Icing" or "Polyester glazing putty", that is your choice, I use both depending on the size of the area being worked. Do not use anything that doesn’t mix with a hardener. NO, “Spot putty” in a tube, only polyester putties or fillers. If it uses a hardener, it cures to a hard film. The “spot putties” stay soft and can become even softer when the solvent from the primer coats it.
You now run a block, long board, or hog even over this skim coat with a little bit coarser paper than you plan on finishing with to cut off the resin that has surfaced in the filler. I usually just use the 36 or 40 or whatever I have been on the "rough" work. BUT take CAUTION not to cut much off, you want to JUST take the very top, don't really sand AT ALL.
Now finish sanding with your longboard or block or hog or whatever using the finer paper like 80 on a large area or 120 on that small 8" sized area. Block it out to perfection with a nice feather edge to the surrounding metal.
I can't stress enough, the trick is to know when just ONE LAST skim coat will do the job. And apply it COMPLETELY over the surface. If you only one little low spot in the middle, DON'T just do it, skim the ENTIRE thing. You HAVE to have one LAST skim coat over the ENTIRE thing every time. If you get in the habit of this you will do it over and over on every dent you repair and find that you can do just about any dent with just two applications.
As you sand the filler let the board or block you are using run over the surrounding metal. If you only work on the filler you will sand it too low. You need to keep it as high as the surrounding metal, so use the metal as sort of a straight edge that you run the block or board off of.
Don’t worry if you cut through this skim coat here and there. In fact, you WILL most likely cut through. The point of that "LAST SKIM COAT" is that after you add it, you don't add ANY MORE filler. That "LAST SKIM COAT" is just that the LAST filler you add. If you hit a little filler below, or metal, that is normal and fine. The only thing you are looking for at that point is if the panel is FLAT. The filler skim coat is serving no other purpose than to finish you filler work, it is not a "sealer" or anything like that.
You can add fiberglass resin (“A” coat if you have a choice) adding the resin was exactly how I learned from the great Emery Robinson (my personal hero in the auto body world). But remember there was no products like polyester putties back then. When you add resin, that resin comes to the top of the film of filler. It is then something you have to deal with. The whole purpose of the SKIM COAT is to put a layer of filler over the top that is easy to block out with as little effort as possible. You want to be able to concentrate on making the panel FLAT not fighting with gummy resin, sand scratches and the like.
So the polyester putty though expensive is what I use.
How is this for an idea, a co-worker of mine showed me this very obvious tip. :)
Add pour-able polyester putty to the regular filler! What an idea! LOL A little pour-able squirted into the "bondo" really thins it out nicely.
The "LAST SKIM COAT" should be left to cure a good long time. Where you may jump on filler and sand it as soon as it is hard, the skim coat should be GOOD AND CURED for an hour or more. If you can of course, in the production shop you may not be able to wait that long. The benefits of the procedure will not be diminished.
A little added note, I have found that I don’t use 36 or 40 grit at all anymore. I went to work at a shop that didn’t use the coarser grits so I had to learn not to also. I have found that using just the 80 and then finishing the Skim coat in 120 or 180 works great, even on large panels.
At this shop it was the first time that I wasn’t doing my own primer work. This meant that I couldn’t “cheat” with a lot of primer and blocking the body work “one more time”. I found that I had to get the work PERFECT, then give it to the painter. I did this in an interesting way, I look at the last skim coat as even a more “final” step. I now look it as “primer”. You see I have used polyester primer, which is like spraying “bondo”. They are both polyester resin based and act and sand very much the same. So, I figured why not just “spread out my primer” as the skim coat! It has worked GREAT, the painter jokingly says, “do you think I’ll need to prime this or just paint it?” I tell him, “Just clear it, it’s a shame to hide that work under primer”.
This method has worked great for me, it’s more of a state of mind than a procedure.
And don’t be afraid to buy the best sand paper and use a lot of it, the cost of the paper will be nothing next to the time and muscles saved. Find the paint store in town that services the PROS the Body shops in town, that is were you will get the right stuff and the right info.
Plastic filler refresher.
I thought that a little refresher on the basics of plastic filler was due.
Yesterday I did two small plastic filler jobs that I thought I would share with you. The procedure I lay out in the “Basics of Basics” for plastic filler is pretty clear, I just thought that a couple of real life repairs might be of help. I also wanted to walk through a repair when things didn’t go perfect. How you get out of trouble is almost as important (some will argue more important) than keeping out of trouble to begin with. The first repair was a small 18x12 inch dent in a door of a Hyundai. I don’t think I have to tell you, the metal is very thin on this car which poses unique problems. I have pretty good access from behind with the trim panel removed to push it out. It did need some shrinking (I used my stud welder with a shrinking tip). The metal was pretty close and I figured one filler coat with “regular” filler and a skim coat with polyester putty should be about it. I applied the “regular” filler (in this case Evercoats Rage Gold). I roughed it into shape using my 8” orbital sander with 80 grit. The panel was ready for my skim coat when I realized it had a problem spot. There was a 5” round area that was flexing. This is a common problem with stretched metal. It was straight (or close to it) when “relaxed” but when pressure was applied while sanding it would flex down. So, the pressure flexed it down and sanded it “flat” WHILE it was flexed. The panel then came up to it’s relaxed state and the filler that is on the area would then be high! This is a common problem with beginners, they don’t notice the panel is flexing and block and block and wonder why the panel is not flat.
I had to shrink it in that area so I sanded off the filler in just the effected area and shrank it with the stud gun until the metal was firm. I knew it was not close enough for my skim coat so I applied a thin coat of Rage, just on that one 5” round spot. The rest of the filler stayed as it was, sanded with 80 grit. When I sanded the spot I found that I had not put enough filler on it. There were a few “shiny craters” showing low areas. Now, it was pretty close and a skim coat “may” have taken care of it. But I don’t like “may haves” I like to KNOW when I apply the skim coat THAT is it. I went ahead and applied one more coat of Rage. I sanded it and it was done. NOW at that point I KNEW it was ready for the skim coat. I applied it and it was then sanded to perfection. If I had applied that skim coat too soon and found low spots not filled, I would have had to COMPLETELY skim coat it again, as shown in the next job. It was a mistake, I should have had the metal ready for filler and it would have been done earlier. But such is life. The other repair was on a quarter panel of a 2002 Ford Focus. It was a crease requiring filler in an area about 22x16 inches. As in the “Basics of Basics” you need to give the dent the respect it deserves. The actual damage before the repair was only about 18x6 inches. The paint was stripped out to about 24x18 inches, you need to be sure you are feathering out the filler unto undamaged metal. After pulling the dent (with an Eagle II electric dent puller, neat tool) I applied the filler coat with “regular” filler. I shaped it out rough with 80 grit just getting the body line that ran thru it close. No super detail, I only wanted the line to “be there” and would fine tune it with the skim coat. After applying my skim coat of polyester putty (Evercoat's “Glaze coat”) I blocked it out. I had two areas that were close, but not good enough. One was a tad low, the other had a high spot of metal from the puller poking up. I couldn’t get the high spot to go down with gentle tapping with the body hammer. If I could have done so, and gotten behind the low spot to tap it out I could have saved it. But that was not in the cards. I went ahead and tapped it down with a punch and hammer (to isolate the force) and skim coated the whole thing again. These areas were just four or six square inches in size but I know that feathering a little polyester putty into the surrounding very thin putty would likely be very difficult at best. I skimmed the entire panel. That’s right, the whole thing again. It is a “shell” of polyester putty. Very easy to block, no feather into existing filler. It is just like a primer over the area and much easier to work with. Even though it is more sanding, it is much easier. Again, a mistake necessitated the 2nd skim coat. But mistakes happen and if I would have tried to repair it without another FULL skim coat, it is likely I would have spent even more time to fix it.
Cut your losses and skim it again, but don’t plan on anything less than a skim coat to finish the job.
I want to say there are times when a tiny amount of polyester putty could be applied to pin holes or other VERY tiny imperfections without skim coating the whole thing. But these should be VERY little imperfections.