How best to thin a varnish?

[paul simonson]

Hi all,

Beginner woodworker here with what is probably a basic question. After testing a few varnishes I have found one I really like but I'm still getting the application technique right (using a brush) and seem to be leaving thicker `blobs` usually around the edges. these are slightly darker and raised up noticeably.

Apart from practicing more, I'm assuming if I thinned the varnish somewhat it would both spread a little easier, find it easier to settle/flatten itself and hopefully help avoid the blob effect I am making.


The varnish is http://www.johnstonespaint.com/paint...r-wood-varnish if that helps.

What can I thin it safely with, making sure I don't compromise the positive/protective effects of the varnish and what ratio should I dilute it by?

thank you

[Andy Giddings]

The website didn't provide any information on whether this is a water based or oil based finish - at least in the US, most cans will have instructions on application, thinning ratios and the correct material for thinning. I suspect as this is a fast drying finish its water based. If you want to try thinning, and there is no info on the can, I would call the manufacturer before trying it. At least some WB varnishes I use can be thinned with water up to 20% which helps a lot when its a higher temperature as the finish flows out better.

Depending on the brush material, you might have better luck with a very fine synthetic bristle (I use Golden Taklon for WB varnish) for a better finish. I've also used foam brushes successfully. I try to avoid build up on sharp corners by making sure the brush is only lightly loaded and use light pressure, otherwise it almost scrapes the material off the brush. Also, avoid going over areas that have already started to flow out, especially if its a warm environment. Are you sanding between coats?

[Rob Young]

"Johnstone's Interior Varnish has been specially formulated to give a hard-wearing and non-yellowing finish that dries quicker and with less odour than traditional solvent based varnishes. It's tough and durable, provides easy to apply, scratch resisting protection, and enhances the natural beauty of the wood."

This implies, to me, it is water-borne. However they certainly make it difficult to find the MSDS or PDS.

If it is a water-borne formula, thinning with water isn't going to much help your brushing technique. Consider using an applicator pad instead of a bristle brush.

[Jim Becker]

I agree with Rob...if this is a water borne product (no actual indication on the website as to what it is), then thinning really isn't an option. Padding is likely the solution, IMHO. If it's an oil based varnish, it can be thinned for better application consistency, but the more you thin, the more coats you'll also need. It's a compromise.

[Doug Garson]

Does the label specify what to use for cleaning the brushes? That should clarify if it's water or oil based. If it's soap and water, then it's water based, if varsol or mineral spirits it's oil based. Your problem is likely more dependent on technique than thickness of the finish (I'm having the same problem with some spar varnish). One tip I saw (can't find the source) when varnishing a flat surface with edges as you describe, is to start an inch or so from the edge and brush all the way off the opposite edge then brush back and brush off the first edge. This helps prevent the build up you describe which accumulates when you drag the varnish filled brush onto the surface from off the edge. Don't think this video contains that tip but here's a good video from an old master. https://www.youtube.com/watch?v=_VdFzj48BOQ