If you need to use heat lamps on top of your part isn't mostly a good sign in best conditions you have your part in a room with very low humidity and constant temperatures (part and resin) at around 23°C the entire cure. meaning not just while it is curing and feeling hard but full 48 hours. 
before sanding it, make sure the resin is "fully cured" not just "feeling hard". I let my parts cure for 72 hours after last pour and even postcure them till 55°C to avoid having the mistakes above. 
by all the above I'm not saying you cant use this resin in your garage or shed with a heater on the part, but it will make it harder