What paint should I use to seal an indoor table to use outdoors?

exterior marine grade varnish

I would use exterior house paint, or paint specifically labeled outdoors.

Hello Shellseeker :) Cute name ;) Anyway, below is a link I am providing about how to weatherproof furniture for outdoor use. It is from a very reliable source ("THIS OLD HOUSE") which is why I am sending it to you (I've never personally weatherproofed any furniture, but maybe I will try it now) Have fun iwth your project and thanks for asking Hometalk for answers :)

https://www.thisoldhouse.com/ideas/how-to-weatherproof-furniture

I would also use an exterior paint and then a coat or three or six of polyurethane to help protect the paint.

I would paint with exterior enamel. Then spray with a coat of Rustoleum Brand clear coat.

Those would be light coats, not heavy. And yep it takes awhile, but it protects longer. Depending on the product, a couple of days, or a week if it says to wait 24 hours between coats. And some require light sanding between each coat.

Hi Shellseeker! :)

If you want to keep the table for more than one season, the best thing you could use would be coat it in Marine Varnish. The same varnish they use on boats so it'll be weather proof. My favorite Marine varnish is this one from Home Depot, but I'm sure you can get it pretty much anywhere.

https://www.homedepot.com/p/Duralux-Marine-Paint-1-qt-Clear-Spar-Varnish-M738-4/205127238?cm_mmc=Shopping%7CG%7CBase%7CD24%7CMulti%7CNA%7CPLA%7CAll_Paint%7c71700000032325895%7c58700003835893712%7c92700030694005904&gclid=CjwKCAjwlcXXBRBhEiwApfHGTR0tyyzdwUj534tA23bwpWk_K1cFvPy4_6h9-AbGMPxbK55icLSKWhoCqOsQAvD_BwE&dclid=COLQ4LSY9toCFcoEDAodnxILjA

Would love to see this when you're finished! Good Luck!