Instead of purchasing dry ice, you can try making your own carbon dioxide from a teaspoon of baking soda and a few drops of any weak acid. Vinegar is the usual choice. A sports water/drinking bottle which has a tube to drink from can be used for mixing the stuff. Just make sure the bottom of the tube is not under your mixture, or the build up of gas will force out the liquid mixture.

Baking powder is a bit more expensive but has a dry acid in it. You just add water. This leads to a thought about whether a floatbag or kayak float could be constructed that would be self inflating, using the gas released from the action of baking powder and water.

Since all the ingredients are edible, as long as you don't get it messed up with varnish you can rinse out the waterbottle and reuse it. In fact, this mix may even do a nice job of cleaning out the bottle!

Other devices for mixing would be a balloon, condom, or plastic bag. If you wish you can tape the neck of any of these around a soda straw and get a bit better control on where the gas flows to.

But if you are gong to mix up chemicals in a plastic bag, why not just use the bag to cover the top of the varnish?

Then there are seltzer tablets and powders. Alka-Seltzer and Bromo-Seltzer. I remember in the days of "Sea Hunt" that Mike Nelson would use small floats that he could inflate underwater to mark the location of items he found. A few years later I saw similar ones that held an Alka-Seltzer tablet. The operating action was to open a valve that allowed an ounce or two of water into the float, then close the valve and hold the float so the Alka-Seltzer tablet was submerged. The gas that was released did a fine job of rapidly inflating the float. To recharge it, you washed it out, dried it, and added another tablet.

Carbon dioxide powered inflatable lifejackets are sold at most boating stores. So are the metal tubes that hold the gas. These containers are also used for the propellant for BB and pellet guns and paintball guns. You might just unload the BB gun and fire a few shots of plain gas into the varnish can.

PGJ