A Quick Guide to Using Body Filler:
If you are doing any type of body work, there's a good chance that you'll be playing with plastic body filler, often called bog. There are 3 basic types of bog, all consisting of a base plus a catalyst (hardener). Generic, lightweight plastic body filler is the one most commonly used for body repairs. It's hygroscopic (absorbs water), so don't wet sand it or expose it to moisture before painting
Fibreglass filler is similar but it is mainly resin mixed with fibreglass strands. It is much stronger than regular filler, making it ideal for bonding things together and filling holes. It's not hygroscopic so it is good for sealing repairs. It is sandable but fibreglass filler furs up at the edges when you try to feather it out. It's intended to have a skim of regular filler over the top to achieve a smooth finish.
The third type of filler is a fine finishing filler for repairing minor imperfections such as hail dents and small nicks and scratches and is the only one that can be applied over paint.
When mixing and applying bog, don't pile it on, It's meant to be used less than 2mm thick(10mm max). Shrinkage does occur, so in thicker applications apply multiple layers and sand inbetween layers. Too little hardener and the bog will take to long to go off; Too much hardener and you will significantly cut down the workable time and create excessive shrinkage and heat-it can get hot enough to warp panels!!!
Mix on a smooth, flat surface and make sure you blend the hardener right through as an unhardened spot of filler can ruin an otherwise perfect job.
Use a wide, flat applicator for mixing and applying filler. Cheap plastic applicators are fine for a one-off job but if you are going to be doing a lot of filler work, invest in a set of decent steel blade applicators. They will last for years if you thoroughly clean them after each use, the same applies for your mixing board , which should be cleaned before the bog hardens on them.
Don't contaminate your bog or hardener. Be wary of dropping hardened bog or traces of hardener into your container otherwise lumps in the bog will make smooth application impossible. Be quick when mixing, You have about 1 minute to mix in the hardener and 3-4 minutes for application. Work time varies by up to 2 minutes with the amount of hardener used and once bog starts to go off, it hardens quickly. Don't continue to apply filler after it goes beyond the thick, honey-like state-it won't adhere.
Apply the filler over a larger area than the actual repair- at least 50mm all around. Never apply plastic body filler over painted surfaces as it won't stick- it can't key properly and will delaminate.
Be quite generous when sanding the paint away around the repair as this allows you to properly feather ( sand smooth the surrounding surface) the edges of the filler. Never attempt to to get all your bog in at once- several applications are normally needed. Initial layers can be roughed down with a cheese-grater file before the filler completley hardens- after 5 mins, until about 15 mins. After that leave all sanding until the filler has fully hardened, at least 20 minutes. happy bogging!!!
The above extract is from street machine, January 2005 issue, page 130. authors : Andrew clatworthy, Craig Parker.
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