There is a product called Tub and Tile by White Knight which is tricky to apply and not a perfect finish but is cheap!! Depending on how good you want the end product to look and how long you want it to last for, there are a few options. I regretted using this myself as the brushstrokes stood out and the paint started to peel off in less than a year.
I ended up having an inner bath fitted (at a cost of around $700!) rather than resurfacing, as the tenants were already in there and couldn't not use the bath for the 4 days required, as the shower is over the bath.
There are new products out which may solve your problem. Best bet is to go to a specialist paint store and have a chat to them. Best of luck!
there is an "epoxy" product specific for bath tubs &/or show recesses (comes in 2 containers & you mix them when you are about to use it) that I had bought from Bunnings earlier in the year... as it is slow to dry, most brush strokes seem to even out... in my case it worked well & 6 months later it still looks good (I asked the tenants, but will see it in my next inspection)... I can't remember the brand, but was in paint section (ask the staff on duty)...