Peparation (sanding/priming) guidelines for composite wood (mainly HDF/MDF/hardboard)ApproveRejectUn-ApproveSubscribeUn-Unsubscribe
Question asked 2017-02-11 12:21:09 ...
Most recent comment 2017-03-20 16:56:00
Grounds / Priming
Industrial and Non-Traditional Products
Rigid Supports
Sizes and Adhesives
As far as preparing rigid engineered wood surfaces for painting goes, the consensus seems to be that they should ideally be 1. cleaned 2. sanded 3. sized 4. primed. While various websites advise sanding according to preference between subsequent applications of ground to achieve a very smooth surface, what I usually found lacking is the advice of sanding before sizing.
My questions are as follows:
1. To clean the surface before sanding, I would use 95% ethanol applied with a rag/kitchen towel. Is this alright? Other options that come to mind are methylated spirits, mineral spirits or hardware store soap, all advertised as pre-paint cleaning agents, but I'm not sure whether they would make a difference and concerned about breathing in the methylated/mineral spirits fumes.
2. There is generally no information about how hard the surfaces should be sanded - only that they should be sanded "lightly" so that they are lose gloss. The problem is that in order to really remove all visible gloss one has to sand much harder than lightly, so I sometimes have fibers sticking out of my boards after sanding them, which is apparently a sign of sanding too hard. Is there any way of judging how lightly one should press while "lightly sanding"? Any way of checking other than just running a finger across the surface (possible health risk?)? Also, I understand that a 150 grit sandpaper is a bit too rough and something in the range of 200-250 would be better?
Finally, is it worth buying an electric sander? I think it might give me a more uniform result, but I'm not sure if it won't be sanding too much even with the lightest touch given the speeds involved.
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Moderator Answer
(Kristin deGhetaldi)
We have some information about this in our "Rigid Supports" document that can be found in the resources section that I will include below. As for sanding too much, it is not the end of the world to have a few fibers sticking up here and there...try to lightly sand those areas down if you can because they can cause skips and breaks in the sizing/ground layer(s) during application. As for health and safety precautions you should always wear an appropriate face mask and goggles when you sand anything. You can find additional information on this topic in our "Health and Safety" document in the resources section. If you do go with a belt sander just be careful that you do not over sand as it is very easy to do so rather than sanding by hand (you will have more control with the latter even though it is more arduous).
"Degrease the surface with denatured alcohol before applying layers of
size, ground, and/or paint. Without this preparation, ground layers may
not adhere to the smooth side of these supports because of surface
resides such as paraffin wax that may be left during the manufacturing
process.
• Gently sand the face of the panel with fine sandpaper (e.g. 220 grit) to
provide a slight mechanical tooth but be careful to not overly roughen the
surface and unevenly expose the wood fibers. This can lead to irregular
swelling of the substrate (particularly when water-based sizes, sealants
and/or priming/ground are applied)."
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Moderator Answer
(Kristin deGhetaldi)
As long as you put on enough size and ground to adequately cover and coat the fibers sticking up you should be alright. It is preferable not to have a tone of fibers sticking up from the surface but if you coat them sufficiently with sizing and priming it is really no big deal...you really want to diminish that sheen to ensure adequate adhesion.
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Moderator Answer
(Kristin deGhetaldi)
Yes that sounds about right to me...if you are an "aggressive" worker by nature you might want to be a bit cognizant of that...likewise if you are someone who does not do a whole lot of manual labor (and therefore might need to be a bit more aggressive when sanding)....
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Moderator Answer
(brian baade)
So sorry. We seemed to have missed your final comment on this.
I do not think that a drill-based sander would be maneuverable enough to work
for this without causing major damage.
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