Originally Posted by noob00224
The plywood is not 100% smooth, but more smooth than the surface of Cinewhite for example. Similar to Cinewhite UHD.
There is a real need to clarify a few points here....so as to avoid @noob00224
from having to discover things the hard way when some correct knowledge will allow him to bypass known issues.
Unless wood is sealed with a Primer, or a Paint that is heavily laden with Acrylic, the application of paint will raise the grain even on a wood surface that feels smooth to the touch. This effect is both well known and universally accepted by Finishers everywhere. When such a sealer is not available, then multiple light coats must be applied, with a light sanding after every one. This is done so as to remove paint applied to higher points while letting lower ares gradually "fill In". This is a highly inefficient way to go about smoothly finishing a surface, but one often attempted by those not familiar with or unable to use correct methods.
Once a surface is filled in to the point where everything is leveled, a last slightly heavier coat is applied, one that can finally be sanded without the higher points (Grain lines in this instance) no long stand out when a sanding material is used.
Unfortunately, in most cases the application of 1-2 heavier coats simply raises the high points at the same time lower points are filled in. As such all the more paint must be removed off the high points just to get back to a starting point...usually by aggressive sanding. If a fine grit Sanding is done on a raw wood surface until it feels exceptionally smooth,that helps of course, but without a proper sealing, the moisture within a paint (or solvents withing a oil based paint) will be absorbed unequally, again resulting in the raising of the "appearance" of Grain.
There is simply no substitute to the proper preparation of any surface. If a ALR-oriented translucent paint of any type is to be effective without unduly attenuating absorbed light, the under-lying substrate MUST be as reflective as possible without itself introducing excessive sheen or or texture related artifacts.
(Artists who use Canvass as a substrate use something called Gesso, a very thick coating that is "spread on" that hides the texture of Canvass)
As such, applying a different ALR-grey coating over a previously applied Grey coating is self defeating...unless one is willing to accept that attenuation will almost always be increased beyond what should be expected / desired / required.
To close this point out...since DIY'ers have been painting surfaces for screens,there are those who wish to experiment with multiple attempts. And naturally do so without expending any more expense or effort that necessary. Perfectly understandable...but nonetheless something that must always be defined and explained dependent upon the circumstances in order to avoid leading to ineffectual or misleading end results. If one wants certifiably correct results on a given surface by which to make correct assessments, short cuts or the use of unadvised methods should be avoided. Do it right, or accept that you cannot make a validjudgement as to performance, no matter what paint solution is employed.
Should sanding be done between the 6-8 coats on the final screen?
The reason why a "very" light sanding after the 6th coat is advised is because even with the most care in spraying on Duster coats, the introduction of multiple layers of "dots" of paint can and often does still introduce a light texture. This texture is so slight that it can be "knocked off" with just one very light sanding, allowing the final 1-2 coats to achieve a much smoother surface. Emphasis must be made again that such sanding is extremely light.
Or is the sanding to remove or smooth out the paint on the plywood when doing the testing?
Both...if plywood is to be used, which according to what is stated below is not the case going forward.
The screen will not be plywood, probably Flexi white, a white screen from Harnkness, or whatever similar screen I can find.
All the prior reasons for doing a light sanding apply for ANY surface used. Any surface.