me and my dad are gonna make a halfpipe in the garden which is gonna be 3'-3'6" high and any tips such as 6 or 8 ft flat bottom steepness etc and is painting it with this paint http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=170613146886&ssPageName=STRK:MEWAX:IT#ht_1570wt_1139 but in a...