Sunday, September 5, 2010

Using bodyfiller

In order to use bodyfiller successfully, it is vital that all traces of rusting, paint (including primers), oils and other contaminants are removed from the surface to be treated. Filler cannot adhere strongly to painted metal, because the join can only be as strong as that between the paint and the metal! If you apply filler over the slightest amount of rust then you can expect it to literally drop out at a later date when the rusting
• spreads sufficiently underneath. If you apply filler over contaminants then you may find that poor adhesion results, or the filler could chemically react with certain contaminants.
If cleaning the metal renders it very thin then you should not use bodyfiller because it will offer little or no strength and furthermore, most types of filler are quite rigid and will be very inclined to loose adhesion to a thin and hence very flexible panel, or even to break up as the panel flexes. The only safe option in this situation is to weld in new metal.
Before using bodyfiller, check the surface carefully for high spots. Whilst you can fill and smooth down low areas, high spots cannot be linished out and must be beaten out before the filling process begins. If there are any deep holes, beat these out as shallow as possible, where access permits. Equipment is available for pulling out dents, and consists of sliding hammer rod to which a number of attachments may be affixed. The attachments can fit through a small hole in the surface of the metal, and the sliding hammer is then used to knock out the dent. If the surface can be made clean and yet remain sound, 'key' the surface with a 36 grit disc, then use spirit wipe to remove any grease or oil contaminants.
Most fillers consist of a thick paste and a separate hardener; a chemical catalyst which accelerates the hardening of the filler. The filler itself usually comprises a polyester resin with a mineral-based powder, which forms a thick paste. Alternatives with tiny metal particles instead of the mineral powder can be obtained today. These offer the advantage of not being porous but might not give such good adhesion as the mineral products, which have far smaller particles. Mineral body fillers are porous.
Mix up the smallest quantity of bodyfiller which you feel you can get away with, and always follow the manufacturer's instructions relating to the relative amounts of filler and hardener. Ensure that the filler and hardener are properly mixed and of a uniform colour. Cleanliness is vital, because any foreign bodies in the filler will simply 'drag' as you try to smooth off the surface.
Apply bodyfiller in very thin layers, allowing each to fully harden before adding the next, and gradually build up the repair to the required level. Do not be tempted to apply one thick layer of filler, because this may have small air bubbles trapped within it which will only become apparent when you begin to sand down the surface. Also, some resins and hardeners generate heat as they cure, and, if you apply too thick a layer, the extra heat generated by the greater mass might over-accelerate the curing process.
Build up the surface until it is slightly proud of the required level and leave it to fully cure before sanding it down. If sanding by hand, then use a sanding block. Electric random orbital sanders and air-powered dual action sanders really come into their own when working with bodyfiller, and can both save much hard work and help to gain better results. The author has found the random orbital e'en ric sander which takes one-third of a sheet of paper gives the best results, because it offers a
large contact area and so helps to avoid sanding the filler into a concave section. The best tool for the job — if you can afford or hire one — is the long bed sander, which is powered by a compressor. However you sand down the filler, always wear a dust mask, because the tiny particles of filler in the air can cause respiratory
problems if you inhale enough of them. Before you begin sanding down bodyfiller, ensure that no engine or transmission components are lying out in the open workshop, as the filler dust really does manage to get everywhere! Always finish off the sanding process by hand using a block.
Most bodyfiller is porous; that is, it can absorb moisture. If the filler is allowed to become wet before it is primed, then the moisture can remain in contact with the surface of the metal underneath, and all of your hard work will have been to no avail! It pays, therefore, to spray primer over a filled area as soon as the sanding is completed. For the same reason, never use wet 'n dry paper wet when sanding down filler!

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