I have welded up a couple of seat boxes, and now intend to paint them with a
two-pack polyurethane. However I have a couple of questions that need
answering first.



Is polyester filler the correct filler to use on aluminium, if so can the
filler be used directly on the aluminium or should the surface be etched
primed after sanding.



All help on this topic greatly appreciated.

Max.

I would put some self etching primer first then the filler then some epoxy
primer then finish coat.
First is the bond to the aluminum, self etching primer applied after
treatment with an acidized aluminum cleaner will bond and provide a non
corrosive base for the remaining layers.
Next the filler, if you use automotive type (bondo) this can absorb water
and if it where placed directly on the aluminum eventually it might allow
moisture reach the al and start those bubbles that you have no double seen
on painted al.
Two or three coats of epoxy primer will provide some build and seal thing
off pretty well.
Finish coat. Actually two part urethane may not be the best choice for this
unless you have this on hand already. You may find yourself spending a bunch
two get it in the packaging it normally comes in. You may be just as happy
with a urethane modified enamel.
To find these products, go to and automotive paint jobbers.
al cleaner............. ~$10/qt
The self etching primer....Dupont 615S............~$45/ (2) qt. unit
epoxy primer .............. ~50/ (2) gt.
finsih, urethane mod enamel ( Limco 1234) ......~70/ qt + stuff
(Enamel, reactive reducer, urethane additive)
I Think the key to painting aluminum is to get good adhesion and build and
weather proof system over that, but then all painting is pretty much that.
ps Limco is a BASF product any other equivalent product will do as well.
--
Ron White
my boatbuilding web site is:
www.concentric.net/~knotreel

Thanks Ron, that has been very helpful.

Regards
Bruce