I use simple acrylic water based varnish. 3 coats maximum or it goes filmy. Leave a few hours to dry between coats.Stops water absorption, doesn't change the look of the terracotta unless you use a satin or gloss finish but won't grow algae for many years either so no weathering.
Read back and then try clear acrylic varnish or PVA glue. I find the varnish easier to apply.