Plastic pipe or sheet should always be scraped prior to welding; this removes any contamination and/or oxide layer (seen as a shiny surface) at the joint interface. This ensures that welding takes place polymer to polymer, to optimise weld strength. Scraping can be done using a bearing scraper or a piece of 1.5mm thick stainless steel sheet, roughly 90mm x 50mm in size. Inter run scraping should also be employed to remove any particles of over-spill, using a 6mm wood chisel. This is to make sure that the weld is built up in a uniform manner. The welding rod should always be wiped with a lint free cloth, to ensure freedom from contamination or dust particles but scraping the weld rod is not normally necessary.
See further information about plastics welding and testing.