The application of Poly-Fiber's UV Smooth Prime has proven more of a
challenge for this non-painter than expected.
In my innocence, I thought I had eliminated most of the pinholes using filler
- none were apparent after vacuuming the surface. Washing with Poly-Fiber's
soap made pinholes appear in a number of places -- apparently, the pinholes
were filled by dust during sanding which was removed by washing.
I applied 2 un-thinned coats using the the suggested foam roller. On drying,
this had the texture of about 180 grit sandpaper. It filled most pinholes
nicely and where the pinholes weren't filled by normal rolling, scrubbing
back and forth with the rounded end of the roller followed by re-rolling did
the trick. However, one or two defects larger than a pinhole - perhaps 1/16
inch diameter - were apparent after the initial coats of paint. A dab of
Smooth Prime applied with a toothpick to these left a little blob proud of
the surface; after drying, the blob was shaved off with a sharp wood chisel.
Save a little smooth prime from the roller tray in a 35mm film container for
application by toothpick the next day but be sure to use it within a day or
discard it since the crosslinked paint deteriorates quickly.
I was unclear on how to get rid of the rough texture so I tried sanding prior
to spraying the next 3 coats. Further trials seemed to indicate that this is
the wrong thing to do: the smooth surface is much more prone to develop sags.
After more trial and error I found that for me the best approach was to thin
the Smooth Prime slightly prior to rolling, about 2 oz of water per 16 oz of
paint. This seemed to result in less texture due to the roller and better
coverage from the initial 2 rolled coats.
One observation on rolling Smooth Prime is that it dries very rapidly, so
rapidly that it thickens around the plastic center of the roller where the
handle enters the roll. This thickened paint will occasionally leave a 1/4
inch wide track when uneven pressure on the roller causes it to be depositied
on the surface. Rolling back and forth over this line seems to remove it but
later examination of the dried surface with a glancing light will show a
raised line. Wiping the end of the roller where the handle enters with a
paper towel occasionally to remove the thickened paint helps to avoid the
problem. Failure to wipe the roller end when painting large surfaces like
the wings can cause "flakes" of hardened paint to be deposited randomly on
the surface.
For spraying I am using a Croix CX-9 HVLP system with 40 feet of hose (to
cool the air) and a 1.2mm orifice with a 0.7M aircap, all courtesy of the
local EAA chapter.
This spray system produces a slight "orange peel" in the finish despite my
efforts to get a smooth finish to minimize sanding. The Poly-Fiber pamphlet
shows the orange peel effect of spraying Smooth Prime in one of the photos
and indicates that it will smooth itself out as it dries. I found that when
applied according to the pamphlet it seems to dry with a definite orange
peel, enough so that it takes a considerable time to sand down to a smooth
finish. The best finish I was able to obtain came by adding more than the
recommended amount of water -- eventually settling on 5oz water per 16oz
Smooth prime. This is much more than recommended and was more difficult to
apply without getting sags but produced much less surface texture resulting
in considerably reduced sanding. I avoid sanding between coats since the
smooth sanded surface promotes sags.
Comments on other ways to minimize orange peel would be appreciated. As a
non-painter, I have difficulty deciding which of the many variables to
adjust. Trial and error with this paint is too costly and time consuming a
way to improve results so some expert guidance would help. In particular,
would a smaller nozzle and/or bigger aircap be helpful? How does Smooth
Prime compare to Top Gloss as far as orange peel susceptibility is concerned?
On sag recovery: there is a "golden moment" when the Smooth Prime around the
sag has dried and the sag itself is still wet. If you can pick this moment
(I've gotten some practice) then laying a paper towel over the sag and
patting it lightly followed by peeling (not sliding) it off will make
subsequent sanding much easier. If a sag or drip has dried, a sharp wood
chisel parallel to the surface will slice the thick portion of the drops off.
Scraping the high areas with the chisel held at 90 degrees to the surface
followed by light sanding will complete recovery.
On the subject of surface defects, I note little grains protruding from the
surface after spraying or rolling. When inspecting the surface with a
glancing light, these grains cast shadows which look a lot like pinholes
until you look closely. The trusty wood chisel removes them nicely. Smooth
Prime seems to dry on the outlet of the nozzle and little pieces break off
and are deposited on the surface. Wiping the nozzle from time to time helps
but doesn't eliminate this. I hope that this is due to the "high build"
nature of Smooth Prime and that it doesn't happen with Top Gloss.
I was originally going to build a wing holder to allow rotating the wing so
that the surface could be held horizontal for spraying. Per Gemin's input, I
just mounted the wings in holders so the LE was uppermost. The wings are
held by a "U" of plywood screwed to a trestle, clamping the spar extension to
the trestle to prevent rotation. An inverted "T" using an 18 inch square
piece of ply on the floor with a 6 inch wide vertical piece braced by 45
degree struts fits into the aileron closeout to hold the outboard end up - a
pad of cloth prevents damage to the closeout paint which was applied earlier.
It is surprisingly stable and allows some sanding in this position although
serious sanding is best done with the wing horizontal.
Suggestions on any aspect of painting would be helpful.
John A044
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