Spit, beer and elbow grease! When that fails, Crown PSCl
Diesel or kerosene work well.
It depends if you are environmentally friendly or not, I try to do my part and use this but it is stupid expensive and evaporates over time. It works OK and is safe but it cost me 600 dollars to half fill my tank.
https://www.amazon.com/gp/product/B003OAAQ7K/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1