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I am in the process of applying polyurethane varnish to a finished workpiece and have been sanding between coats as advised by the manufacturer instructions.

I initially wondered why this was necessary until, after applying the first coat, I noticed that the wood was bumpy where it hadn't been previously. Why does this happen?

Am I doing something wrong? My assumption is that sanding between coats ruins the finish.

I used this varnish: http://www.woodoc.com/products/24.

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  • You need to specify exactly what type of varnish you used as "polyurethane varnish" could be waterbased or oil-based. If one assumes one or the other the advice given would be completely different ....as in the two Answers you've already received :-) – Graphus Jun 5 '16 at 8:38
  • I used this: woodoc.com/products/24. It isn't water based. – user2251 Jun 5 '16 at 11:54
  • Did you manage to figure out what the source of the rough surface after drying was? Is it dust or was the varnish overworked when being applied or something else? – Graphus Jun 8 '16 at 7:24
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    I still am not sure really. I tried going easier on the coats with the following pieces I finished, but they had the same issue. I was concerned that it'd still be rough after the 3rd (final coat), but the problem seemed to diminish after 3 coats, so after the 3rd one the finish was smooth. – user2251 Jun 8 '16 at 8:19
  • @Graphus Having done another coat today, I think that the roughness is dust that is adhering to the varnish. A very light sand with 220 is adequate to remove it. I have been more aware of the issue and done my best to ensure that the application and drying is in a different environment to the sanding one. That, and a good wipe down to remove dust, seems to do the trick. – user2251 Jun 8 '16 at 16:21
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Usually when varnish makes a surface rough, it's either that it's picked up dust during the drying process, or it was being brushed/wiped too long after application.

Gently sanding between thicker coats is necessary to ensure the peaks and valleys of the finish don't interfere with adhesion of future coats, and so their appearance isn't exacerbated by future coats. But keep it gentle, and don't be afraid to use a higher grit than the manufacturer's specs.

I highly recommend thinning. Thinning a varnish to wiping consistency (about 50/50 with OMS) and applying twice as many thin coats (that dry twice as fast) eliminates a host of issues with finishing.

Fix: ensure the drying environment is relatively dustless, and don't overwork the application (wipe and say goodbye till it's dry)

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    There are actually no issues with varnish adhering to previous coats with nearly any varnish or water-based finish . Although this is often (too often!) given as a reason to sand between coats it's bogus, varnish will stick to itself with almost 100% reliability with zero sanding. – Graphus Jun 5 '16 at 8:41
  • You're right, varnish will stick to itself nicely without the sanding. Might as well level out the peaks while you fill in the valleys, though (if you're going for appearance.) – Paul Nardini Jun 7 '16 at 19:54
  • With oil-based varnishes if the surface is rough enough that you really need to flatten off between coats via sanding then something about the application needs to be addressed :-) – Graphus Jun 8 '16 at 7:22
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The bumpy feeling you describe is probably the wood grains raising up after applying your varnish; especially if it's a water based one. I previously asked a question about why it's important to raise the grain.

Sanding between the coats shouldn't ruin your finish. Just make sure you use a high grit count and sand lightly. A typical rule of thumb is to use the next grit available after the last sanding grit you used. (For example, if you finished your piece with 220, then it should be 320.) Furthermore, sanding between the coats allow you to remove minor imperfections such as bubbles.

It's a bit late now, but I suggest you try on a scrap piece of wood before the real piece. That way, you can see if you like the end result before wasting time and effort on something you are not satisfied. It's also a good practice for the real deal.

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    Not being overly generous with the first coat can also help. – Chris H Jun 4 '16 at 20:48
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    Thanks for the tips! I have been using the same grit (220) for between coats so I'll go with a higher one. At least, I don't think the finish I'm using is water based. Instructions say I must clean my brush with mineral turpentine. product: woodoc.com/products/24 – user2251 Jun 5 '16 at 6:17

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