Hello "Do it Myself" Girl and thank you for your question.
I spoke to Rust-Oleum Technical Support who confirmed that you will only need to completely clean the bathtub as per the instructions. The product label is found here: http://rustoleum.com/cbgimages/docu...
The epoxy is self leveling and you will not require a special sealant over the porous areas.
You mentioned that you plan to spray the product. You will need to use either 100% isopropyl alcohol (not 99%) or 10% toluene in the sprayer – and be sure to avoid over spraying.
Another note: allow the first coat to dry at least one hour before applying the second coat. A third coat can be applied 24 hours after the second coat. Do not apply more than 3 coats. Allow the bathtub to dry 3 days before exposing to water.
Best wishes on your project.