Hi again, there are some previous threads on varnishes that you might like to look at.
Choosing the finish:
Some basic varnishing pointers:
Matt varnishing questions:
Some tips on how to varnish:
Some considerations on when varnishing is and isn't suitable:
Anne has already made the decision to varnish but for others there are a couple of points one needs to consider to begin with. The first is whether you want your work to last, if you do then the surface needs to be protected in some way. If you sell your work to ignore this is shortsighted as it's impossible to know just what will happen to something once it is out of your hands and frankly does a disservice to your buyers. The two options for protecting the painting are varnishing and framing behind glass. Even if you don't think the work will ever be scuffed or otherwise physically damaged (and how can you predict this?) dust is a major problem for acrylic paint because of its low glass transition temperature, the point I was trying to get across later on in the first thread above. If you don't like the unifying effect of a varnish layer then frame behind glass, your paintings will thank you for it in years to come
If you elect to varnish, then you need to decide whether it should be removable or not. For fine-art work there really shouldn't be any question as if the varnish becomes scuffed, yellows with age, cracks, is damaged by dust, or there are any other unforeseen problem it needs to be removable, otherwise your painting is ruined (which is why polyurethane varnishes aren't suitable for use on paintings but are perfect for craft applications). For acrylic this means that a spirit-soluble varnish is really the only choice, since mineral spirits don't really harm dried acrylic paint their removal won't damage the paint layer; an isolating coat of acrylic medium underneath the final varnish is still a good idea, as some manufacturers now recommend. Older types of acrylic varnishes were much like the paint itself so their removal posed a big problem for the obvious reason - the solvents necessary to dissolve the varnish also worked just as well on the paint itself!
Golden, Liquitex and Winsor & Newton all make spirit-soluble varnishes now and there are others. Even traditional dammar varnish intended for oil paintings could be used on acrylics in a pinch, although in the long term (30 years+) its removal will become increasingly damaging to an acrylic painting.
The second thread above mentions Golden's website and Liquitex's Acrylic Book PDF, both of which are well worth checking for information on varnishing acrylics and many other topics. Golden in particular are dedicated to ongoing research into the long-term care of acrylic works so their site is worth going back to periodically to see if they've posted any new information. In addition to these, W&N's site now has a good overview of the basics of dealing with acrylic, you can view this there with a Flash-enabled browser or download the PDF file here
to refer it at your leisure. They have a few more as well on a some other aspects of acrylic use that are worth reading.
If you're left with any other questions give me a shout.