Im sorry, but i cant really understand what you want to do. Are you asking about rust proofing your engine bay( firewall, frame horns, bumper brace/bracket, chassis?) Or just under your hood(which is usually covered with some sort of heat resistant felt material(?)... Check the engine for any oil leaks, if its just overspray or you have busted head gasket issue or leaky distributor (for cars that still has distributor). Anyway, if it was rust proofing, there is POR-15 (black in color), or eastwood rust proof products that you may consider if you want a total RUST PROTECTION.
For a good wash you'll be needing a piwer wash. For cleaning those oily, greasy engine walls, i suggest you cover all electronic/electricals (such as fuse box, battery terminals, starter/alternator/ ground terminals, oxygen sensor/maf sensor and most sensors), spray some simple green or MARINE CLEAN (por-15 product) to oily areas then go crazy with the power washer.
Just correct me if what i have suggested is totally not applicable to your query.