1) Carry out all required straightening, welding, and grinding on the
panel.
2) Sandblast with fine silica sand (if it's still legal in your area).
This will produce a nice smooth finish without too much sandblast
texture (if the correct nozzle and pressure is used). Use a proper
respirator.
3) Spray with 2-part self-etching zinc phosphate primer/sealer. DO NOT
USE A SANDING PRIMER on the bare metal.
4) Use a little polyester body filler to fill and smooth any minor
imperfections that you may have missed prior to sandblasting. It sticks
tenaciously to a well-adhered primer/sealer. Sand it as smooth and level
as possible. If there are any remaining pinholes or scratches, blow the
dust out of them with compressed air, then apply a thin layer of
polyester filler and resand. Personally, I don't care for lacquer-based
spot putty, which seems to stick best to a lacquer-based sanding primer
rather than body putty. Others may have had better luck with spot putty
on polyester putty, but this just reflects my own experience.
5) Optional: IF you want, you can apply a coat of sanding primer at this
stage, then wet sand it to perfection. However, remember that sanding
primers usually have talc or other additives and tend to be rather soft
and porous. I prefer to stay with primer/sealer, which CAN be wet
sanded, although you have to wait longer for it to cure before sanding.
Primer/sealer doesn't fill pinholes or deep scratches as quickly as
sanding primer, hence it would require more coats to do the same amount
of scratch filling, but I think it's worth it.
6) Apply an overall coat of primer/sealer.
7) Apply two coats of a compatible paint.
Notes:
I haven't tried every paint system on the market, so there may be
exceptions to my recommendations. This approach has worked very well for
me, although there are certainly other approaches that would also work
well. Ask your paint jobber for information about paint systems,
preferably those that are easy for the do-it-yourselfer to apply.
Apply each coat within the recommended time "window." The underlying
paint must cure for some minimum time before respraying, but if you wait
too long, you will have to rough-up the surface to insure proper
adhesion. This isn't hard to do (I use the fine gray ScotchBrite pads),
but the problem is that this will tend to cut through the primer/sealer
layer, creating opportunities for moisture to contact the metal. If you
have to rough up the primer surface, I recommend applying a thin overall
coat of primer/sealer before painting.
Paint in a well-illuminated, dust-free area, at the right temperature
and humidity.
Use a thinner that dries at a rate that is appropriate for the
temperature/humidity and your painting style. When in doubt, go with a
slower-drying thinner to help guarantee a continuous "wet edge" but
remember that it may be a little less forgiving in terms of runs in the
paint.
As to what types of paint are best, as long as the paint is compatible
with the primer/sealer, it should work fine. Remember, the primer/sealer
is your most important barrier against moisture and rust. You can put
the world's best paint over a bad primer job and you'll end up with rust
problems or other difficulties.
You can have the primer/sealer tinted to match the final paint color if
you want, but I would think that most solid, non-metallic colors (i.e.,
most military colors) would have enough pigment content that two coats
of paint will be very adequate to completely cover any color of primer.
In addition, if you get any deep scratches that cut into the primer, a
contrasting color will tip you off and let you retouch the damage before
rust can take hold. I generally use a medium-gray or red primer and even
the first coat of OD covers it completely.
If you are using a flat color as the final paint surface, I suspect that
it might not be a bad idea to apply a gloss or semigloss coat under it
for better moisture protection. I think that I would do this if the flat
sheen in the paint is created by an inert filler, such as
talc/clay/gypsum etc., but if it's the result of a chemical additive
that just creates microscopic wrinkles in the surface when the paint
cures, then it probably isn't necessary. I may research this further, as
I've heard many conflicting reports.
Obviously, follow all manufacturer's and regulatory safety precautions
for fire prevention, respirators, ventilation, etc. etc.
Have fun,
Alan
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