Try your best to knock down any high spots and spikes. Then use an automotive body filler. Something like Bondo or Rage. I prefer Rage Extreme or gold but it is not as easy to get as the Bondo product. Rage can be purchased at an Autobody supply shop and some Napa's. Bondo can be bought at nearly any auto parts store and home depot. Follow the instructions on the can and take your time spreading the product. The smoother you spread the product the easier it will be for you to sand.