Varnishing oil paintings

It certainly has come up on many occasions.   I can’t give you the technical reasons, however,  I’m sure Robert will be able to, but I would absolutely not go along that route. 

Noooooooo don’t .  Get a proper artist quality varnish. Windsor and Newton Galleria is fine though there are several others on the market I know Daler Rowney also do their own and I’ve used that as well.  I would imagine “varnish” for furniture etc is much too heavy and cloggy .  I like Gloss ,  Matt or Gloss are quite personal taste.  If you glaze a painting ( under glass ) it is quite shiney hence I like a gloss varnish , the colours suddenly sing and glow.   It’s quite fun to do just wash your brush well afterwards.   Just seen your last comment.  I have never had “crackle” .  I do know some people like that effect, if you do I suggest you Google it. 

I've used acrylic paints on a childs wooden stool and then varnished it with regular yacht varnish. But on a painting I've spent hours and hours on, definitely not! Like Sylvia says, stick to the varnish made specifically for paintings.

In short - no.  At greater length, no, no, a thousand times no. B & Q varnish could be anything - ketone, acrylic, containing various solvents, or even oils, resins.... So I don't know what "clear varnish" might mean, technically.  Its drawbacks are likely to be many and various, but the most obvious will be premature yellowing - that doesn't matter on furniture, for example, but it certainly matters on paintings.  It is also likely to be much thicker in consistency.  It may not be removable - so once it does yellow, you're stuck with it.   Any varnish is at risk of cracking if a) you apply it too soon - 8 months to a year is best, and I don't varnish before a year is up, b) you apply it too soon on a stretched canvas, which hugely increases the risk (because it's flexible).  Artists' quality varnish is available in several forms, and isn't that expensive, because it's never a good idea to use too much of it - if you do want a matte finish, good advice is to apply a gloss varnish first; let that dry thoroughly; then apply the matte varnish - in both cases thinly.  There are also 'satin' varnishes - can't recommend them as I've never tried them. Plenty of variety on the usual artists' suppliers websites.   Read the product description carefully. Craquelure is another question altogether - presumably this isn't something you want, but if you do (and on your own head be it...) there are specific products which will give you that.  The wrong varnish - especially on oil paint - could certainly crack, but not in a way that the lovers of craquelure would appreciate: it's a way of ageing paintings to look as if they were painted a century or two ago - wouldn't fool an expert, but I suppose some people just fancy doing that as a bit of a gimmick, or if they're producing a painting for a film, or even theatre play, that's supposed to look old.