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Welcome to the Bunnings Workshop community @manskersmaking. It's fantastic to have you join us, and many thanks for your question about coating coasters.
If the coaster is entirely painted and no high-gloss glaze is showing, then the Rust-Oleum 298g 2X Ultra Cover Gloss Clear Spray Paint would certainly work. If there were any gloss glaze exposed, I would have concerns about how easily the coating might scratch off.
Please let me know if you have any questions.
Mitchell
People often make coasters from tiles @manskersmaking, which typically have a glazed coating applied through firing in a kiln. This super slick surface does not readily accept paint or coatings well, as it's non-porous. Your ceramic coasters might not have had this coating applied as you'd find on a standard bathroom tile. If you place a drop of water on the unpainted ceramic tile, does it absorb in at all or sit on the surface with no absorption? If the droplet doesn't absorb, the tile is likely coated. Feel free to post a picture of them, and I can take a look. Let me know if you need a hand uploading images.
Depending on what sheen level (glossiness) you'd like on your finished coaster, you can get the clear topcoat in both a gloss and satin (less glossy) finishes.
Mitchell
Hello @Ronel
Welcome to the Bunnings Workshop community. It's sensational to have you join us.
As Mitchell has mentioned it will depend on the tile that you are using. If it has a high gloss finish on it, the clear gloss paint might scratch off the surface. A porous unfinished tile will absorb the paint and provide you with a good seal. Another factor to consider is if the picture covers too much of the tile, the sealer will end up covering the photo paper and none of tile.
If you need further assistance, please let us know.
Eric
Welcome to the Bunnings Workshop community @Zuko. It's wonderful to have you join us, and many thanks for your question about sealing coasters.
Rust-Oleum 2X will hold up against water condensation, but it's always a good idea to wipe off any moisture as soon as possible to avoid water marks or potential damage over time. Condensation left on the surface for too long could leave a slight ring or discolouration, so it’s best to address it promptly.
For the number of coats, two coats should be sufficient for solid coverage, though if you want extra protection, you can apply a third coat. Make sure each coat is fully dry before applying the next.
As for curing time, I recommend allowing the coasters to cure for at least a week before placing anything hot on them, just to ensure the finish has fully hardened and is as durable as possible.
Please let me know if you have any questions.
Mitchell
This was really helpful.Will start as soon as my acrylic paint dries well on the coaster. Thank you so much for helping out upcoming artists like me. Appreciate all the help and service Mitchell 
