Before starting to apply any finish to your model, it is
suggested to do a 'test' on scrap wood of the same type
to be finished, its a good way to see how the final finish
will look before you apply it to your masterpiece!
There are many wood finishing products available, they all have
their good and bad points.
The most common finish types:
- Polyurethane - Comes in multiple forms:
interior, exterior, and pre-coloured. By its nature it is designed
for interior use only, with no exposure to sunlight.
Polyurethane in its usual form is a 'hard' finish. As wood
moves, the polyurethane shell does not, therefore causing
small cracks in the finish. Not that these cracks are not visible
at first. Over the course of years the cracks will enlarge and the
finish will literally peel off. Using it on a model is not really a
concern unless your model is exposed to sunlight which will
increase the speed of the finish failure. One other thing that can
speed the demise of your finish is the combination of sunlight and
humidity change such as sitting the model in a window. The UV
exposure will speed the breakdown of the polyurethane and the
direct sunlight exposure will cause the model to expand and
contract unevenly.
- Exterior Polyurethane - Is designed to be
used on an exterior application, it is not UV proof, only UV
resistant, but is good to use for a model as it is a 'soft' finish,
meaning that the shell moves, not in the sense that it runs, but
will it move's with expansion/contraction of
wood.
- "Wipe-on" Polyurethane. All Polyurethane is
wipe-on, it may be applied by brush, cloth or spray/aerosol. The
method of application determines the drying time and the end
result. Spray-on is good for quick re-coat times, but takes longer
to build up a protective finish. As a rule, spraying loses 50% of
the material into the air, not counting how much you have to dilute
it to spray. Application by brush has better coverage and
polyurethane, as a rule, is self levelling, meaning brush strokes
go away. The wipe-on poly's that need attention are the
pre-coloured type's, the problem being that due to the pigment
added, the amount of polyurethane is less, and to darken the
colour, multiple coats are needed, with steel wool used between
coats. The steel wool gently roughens up the surface so the next
layer of polyurethane will adhere to the previous coat.
- "Quick drying" Polyurethane. These are good
for a quick re-coat time, but, you will need to coat at least three
times to get the same coverage that one brush coat of normal
poly will give, this is due to the amount of polyurethane versus
the amount of carrying agent.
Type of Polyurethane
finish - Gloss, Semi or Satin &
Matt.
- Shellac. Shellac is an exceptionally
'soft' finish also being affected by oils in the hands that degrade
the finish, and therefore is not ideal for a model.
- Stains - There are very few stains that
are UV colourfast, but all are UV fade resistant to some degree. UV
resistant means will it fade when exposed to sunlight or artificial
lighting, the amount of fade is directly proportional to the amount
of light that the wood receives, the more powerful,
the lighter the colour change. If you plan to stain your
wood, before you do, rub it with denatured alcohol, this
will give you a very good idea of how the wood will appear
when a clear finish is applied.
- Wax - It protects the wood, doesn't show
fingerprints and can be made as shiny or dull as you desire. To
apply wax, thin layers are better, and the more shine you want, the
faster you have to rub. It is the friction against the surface of
the wax that creates a high shine. To get that "bottomless" shine,
multiple layers of wax shined to a high gloss each time will
achieve that effect.
- Painting over other finishes - If you
apply polyurethane, or any other finish, to your wood and then
decide you want to over-paint it, you must first key the surface so
that the paint will adhere. This can be accomplished by using
a 220 grit paper. This will assist in the paint adhering to
the previously applied finish, though try a test piece first,
as there still may be some adherence problems.