Kitchen worktop joints - what's the best way to make them waterproof?

You can buy steel clamps consisting of a long bolt to make the joint. You need two clamps per joint. You need a large drill to make the blind holes for the joint. They work very well and allow a small degree of adjustment. Before the clamps were available I used dowels for the job but the clamps make it easier to be accurate and also pull the joint up tighter.

I have just used PVA glue in the past.

In article , no snipped-for-privacy@thanks.com writes

If you have the time I'd say you want first to use sanding sealer or thinned polyurethane varnish in repeated applications over a day to get a good soaked in depth of water resistance. Then use the slow set poly you mention or one of the double purpose sealants such as Plumbers' Gold (both evilly sticky and an excellent sealant) when doing the jointing[1].

Water will have years to attack so it's best to have 2 lines of defence.

Absolutely and make an extra effort to seal the cuts for the biscuits (more area for moisture attack) and fully back fill with sealant on assembly.

Ideally seal the cut end thoroughly first with something that can soak right in and make it non absorbent. The use a generous helping of glue/filler/sealer when assembling the joint. Cramp it up tight with the dog bone connectors.

Yes I normally do - it makes it easier to keep it all aligned while you do up the dog bones. The alternative it to clamp a couple of bits of scrap wood across the joint to maintain the top surface alignment.

Would anything that soaks in be liable to cause local swelling?

I would avoid water based things.... shellac sealer would be good, as would a traditional oil paint or primer.

Shellac needs but a whiff of anything alkaline (bicarb even) to become totally water soluble. I'd go for a moisture curing polyurethane adhesive