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Note: including a lot of details to avoid XY problem.

Background
I have been dealing with a minor pest (psocids, in case you're wondering, but I don't want the conversation to become about that), mainly in a kitchen cupboard and the surrounding area. I've already taken a few appropriate steps, and the next step is to try to close various gaps that the bugs use to go through, such as the ones in the picture below at the back of the cupboard (I think they've nested somewhere behind there). I will be doing that with silicone sealant.

Gaps at back of cupboard.

Problem
These bugs crawled everywhere, including the holes in the chipboard of the cupboard, which they seem to like. One such place where the chipboard is exposed, is the side of the shelves inside the cupboard.

Untreated chipboard at side of shelf.

I want to seal these sides to stop bugs from hiding in there and kill any bugs or eggs that are possibly still alive inside. I don't care much about appearance since these sides are out of sight, but a clear or white layer would be better.

I tried spreading a thin layer of silicone on one shelf, but this makes the shelf a bit "sticky" on the sides of the cupboard. This is not a huge problem since the shelves don't move regularly, but the silicone is coming off easily due to this "friction".

Chipboard covered with silicone layer

I thought to use some varnish instead, but then I saw a guy on Youtube (https://www.youtube.com/watch?v=FV-bLLVrakM) claiming that if I put plain varnish, I risk ruining the shelf by causing it to swell. He suggests "semi-transparent" (i.e., one that includes some paint), which I could not find on Screwfix or other places (UK). His interchangeable use of "varnish" and "stain" does not help either.

So, is it true that I should not use a varnish directly on the chipboard? If so, could I use a coat of paint directly and then varnish on top (since I can't find a mixed product)?

  • The problem with chip board is moisture - moisture it swells. You might try a resin based product to seal it. For example and acrylic or an epoxy compound. Acrylic is used for remodeling old counter tops and perhaps that material might help you the best, as it will not be so much absorbed by the chip board but rather seal it and form a barrier. – Ken Jan 18 '18 at 7:10
  • @Ken, I thought normal varnish was based on a type of resin. – Ratler Jan 18 '18 at 7:37
  • The problem is that kind of resin and the liquidity of the varnish as there are other components - thinners and solvents - so when thinking about liquidity and how easily it is to be absorbed by the chipboard - like water. This is the real issue - what will the chip board most likely NOT absorb. A Plastic resin or acrylic will be too thick to be readily absorbed by the chip board. You could check out Rust Oleums Water Proofing Sealer (acrylic) , Flex Seal or even wood glue. (Keep in mind some bugs are attracted to certain glues..) – Ken Jan 18 '18 at 7:47
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By Googling for "chipboard sealant" I found a UK vendor of "work top edge sealer" that seems to be designed for that sort of job.

  • Seals & Waterproofs Sawn Or Drilled Edges, Joints, Sink Cut Outs etc.
  • Excellent sealing and waterproofing properties.
  • Ideal for use on MDF, Chipboard, Timber etc.
  • Prevents water ingress into sawn edges.
  • Easy to use and fast drying.
  • This is similar to what I used when we had some rentals that had composite cabinets to prevent dampness from swelling in both the kitchen and bathroom the stuff was solvent based and it held up for many years and passed inspection when I sold them (a duplex). – Ed Beal Feb 8 '18 at 23:31
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I've used common water-based polyurethane on particle board and fiberboard with no ill effects. If you allow it to dry in a timely manner, you'll see virtually no swelling. If you want to be extra cautious, use oil based polyurethane. It won't absorb into the particle board as much.

  • I was always afraid to use water based ,glad to know it works also. – Ed Beal Feb 8 '18 at 23:32
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Compiling information from Ken, RedGrittyBrick, isherwood and my own experience to form proper answer:

It's possible to varnish chipboard, but a suitable varnish must be used. Suitability will depend on how difficult it is for its components to be absorbed by the chipboard and cause swelling: oil-based would probably be safer than water-based.

The safest option would probably be to look for a varnish that specifically says it's suitable for chipboard.

  • I ended up using Ronseal polyurethane gloss varnish that says it's suitable for chipboard. So far, so good (I'll update this if something happens). – Ratler Feb 8 '18 at 22:56

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