Lightweight body filler
Used for finer details, a lightweight body filler is used. Many autobody technicians and hobbyists use a lighter filler to use as a second coat over the general purpose auto body filler. It spreads easily, covers with a thinner layer and usually sands a bit easier as well.
Many restorers of classic mustangs also use this lightweight material, applying it as a skim coat to smooth out minor imperfections from door dings, shopping cart damage and other minor issues.
Along with easier sanding, they don't have that tacky skin on the surface that quickly clogs your sandpaper.
Manufacturers of lightweight body filler
Some of these lightweight fillers are manufactured by the following manufacturers.
If you're new to body work, try several brands and find the one that works best for you and the tools you have to work with.
Some products seem to work better with high speed air tools, while others work well with hand tools like sanding blocks and long board sanders.
Common lightweight filler problems:
1. Clumping or partial hardening of the filler:
Typically this means that you didn't mix the filler and the hardener thoroughly enough. Some of the filler is hardening faster then the rest and in extreme cases, some is not setting up at all.
If its the former, you can probably wait a bit longer than normal for it to fully cure. As long as you worked some hardener into the less cured parts, this, or adding some heat from a heat gun or hair dryer will probably do the trick.
If the latter is the case, you will probably need to scrape off what you can and sand off the rest and start again. (sorry)
2. Filler sets up too fast:
In the next batch, reduce the amount of hardener and mix well.
3. Grooves appear as I'm applying the lightweight filler:
You probably have a nick in your spreader, or a piece of hardened body filler that's attached itself to the leading edge of the spreader or spatula. Either clean it off, or buck up and get another one.
Don't be Cheap!
This stuff isn't very expensive any how, but going with the cheapest lightweight filler can be a mistake for a number of reasons.
Look for a filler that is "tack free" or as close to it as you can get. What is tack free? It means that when it hardens, there's no sticky residue left on the top surface to clog your sandpaper and waste your time.
For the longest time, all I would use for second and skim coats were some very expensive products from Europe. What attracted me to them, was the fact that they hardened in two stages.
First, as they cured, they were solid to the touch, but could be sanded very easily, like drywall mud. It was great for quick finishing and evening out small irregularities.
And when they dried fully, they were as hard as rock. Unlike most fillers though, they didn't waste you first several sheets of paper, jamming them up with a film of uncured polyester resin.
It quickly mad my work better and faster.
Fortunately there are new fillers on the market that work as well as those expensive European products. They are a bit more expensive than the garden variety ones, but give you quick dependable results.
One of these is Evercoat Rage Premium Lightweight Body Filler It works like the high end Euro lightweight body filler and I find it saves you money even in the short run.
That's because it spreads so well that you're not wasting it by applying extra material to get full coverage.
It saves steps too. You're pretty much assured of not needing a cheese grater for general shaping and excess removal or a bunch of coarse grit sanding sessions.
The European Standby
Herbert's Standox is one of those European fillers I was talking about earlier. It sands well. It's tack free when it cures. But It's many times hard to get.
Either one is a good choice for skim coats and as a good overall lightweight filler.
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