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  • 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
    Question
    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.
Answers and Comments
  • EditDeleteModerator Answer

    ​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).​"

    Kristin deGhetaldi (CAS)
    2017-02-11 12:56:53
  • ApproveRejectUn-ApproveUser CommentSo is there a way to tell how much sanding is enough? Usually sanding lightly doesn't really remove the gloss from the HDF I use (there's only a couple scratches visible when looking at an angle), but the only way to really make them matte is by sanding so much that the fibers start sticking up. Should I assume instead that the invisible abrasions from light sanding are enough, even if they don't completely remove the gloss?
    2017-02-17 14:55:54
  • EditDeleteModerator AnswerAs 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.
    Kristin deGhetaldi (CAS)
    2017-02-17 15:03:47
  • ApproveRejectUn-ApproveUser CommentAlright, thank you for the advice.
    2017-02-21 11:28:52
  • ApproveRejectUn-ApproveUser CommentI'd like to confirm a piece of advice I've read on another forum regarding sanding - supposedly "light sanding" means that the weight of an average person's hand dragging the sandpaper is enough. Is that true?
    2017-02-22 13:41:09
  • EditDeleteModerator AnswerYes 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)....
    Kristin deGhetaldi (CAS)
    2017-02-22 14:48:13
  • ApproveRejectUn-ApproveUser CommentAn initial quick light sanding with 150 grit will open the surface, follow this with a damp wipe to raise any fibers, then finish with the 220 grit. Never use a belt sander these excel at stock removal and this is not what you`re after, use an orbital sander instead. Never use worn 150 grit as a substitute for 220 grit, it will scratch. The damp wipe in between helps to knock down any fibers that would otherwise stand up during any water-based product application.
    2017-02-25 13:37:15
  • ApproveRejectUn-ApproveUser CommentThank you for the advice, everybody. Regarding orbital sanders - what's the recommended speed (rpm)? They all seem to be pretty high (10000 rpm), so I was wondering whether an electric drill (700 rpm max) with a sanding disc attachment would be easier to use without destroying the surface.
    2017-02-26 10:14:02
  • EditDeleteModerator Answer

    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.

    Brian Baade
    2017-03-20 15:07:04
  • ApproveRejectUn-ApproveUser Comment

    ​As someone who's made panels for many years, I don't believe it's necessary to sand each application of gesso smooth before applying the next coat.  Expending energy to get a layer really smooth, only to cover that smootheness with another layer of gesso and brushmarks, never quite made sense to me (however if there is a good reason for doing so, I'm all ears...).  If significant bumps of gesso, or hairs etc. are stuck in a layer, then for sure I quickly sand to get rid of those - but mostly I just let one layer sit atop the other.  Only at the very end do I do a final sanding, to whatever degree of finish is desired.  It's a small thing, but panel making takes time - no point adding onto the process.

    As noted, a belt sander will remove all your gesso, and I find orbital sanders leave swirl marks.  My preference is a vibrating sander - they work well, but be careful not to rock the pad and dig into the surface.  The dust generated is micro-fine and insidious, and some chalks contain silica (which can lead to silicosis) so wearing a good particle dust mask or respirator (rated N95 or higher) is important.

    Koo Schadler

    2017-03-20 16:56:05
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