OPC works very well below the door sills and under the vehicle with an MF, mitt or MF applicator. If you have problems with Tar down there, I may use either Mineral Spirits or a tar remover as needed. I may also use a prewax cleaner at times.
I keep an old synthetic mitt for cleaning wheels and along the bottom of the car and just use regular car wash or prespray the area with diluted ONR (clay lube concentration) most of the time (UWW+ or OOC would work great here too). Remember that OPC, Tar removers and Mineral Spirits can remove wax from the paint when you remove road tar and grease, so you may need to wax/seal after cleaning this lower body area.
If you meant the door sills and inside the door jams, I provide additional info below.
I moved to OPC about 2 years ago to remove grunge inside the door jams at either full strength or 1:6, when really dirty or when detailing other people's cars for the first time. OPC worked better for me than APC+ or regular car wash. I use OPC in the engine compartment as well. I use a Zymol horsehair brush, "extra soft" toothbrush, Daytona mini wheel brush, mitt or MF towel depending on the situation. I rinse with hose water at low pressure.
Once clean, the easy to get to areas get sealed (been using DGPS for the past couple of years) or waxed when I detail the painted exterior. I normally use OCW instead of a QD to add protection in the tight spaces since it is sprayable. A QD and MF will also work wonders if the jams are kept clean, but add little protection. Don't forget to treat the rubber seals around the doors with a dressing when doing heaving cleaning with OPC or an APC.
I sometimes use a prewax paint cleaner or a light polish to remove hard to remove grunge and heel marks in the bottom of the door ways and to prep the paint before using a sealant or wax (same at the interface of the body and undercarriage). Sometimes residues build up in certain corners around the doors. OPC normally takes this off but sometimes you need a little more persuasion from a prewax paint cleaner (Four Star, OG, Pinnacle, WG, Ultima, etc.) or light polish by hand.
Last but not least. The rubber of neglected older cars tends to build up oxidation
which turns a MF or mitt black when cleaning. I find that OPC (full strength) and a MF towel will remove a great amount of that black oxidation from the rubber before treating with a rubber dressing (OPC and MF towel or applicator, rinse with water, dry and dress).
If you can't get certain grime out of your mitt, I have used Dawn Power Dissolver spray at times (I got mine at WalMart) to get stuff out of the synthetic mitt, old MF and cotton terry towels and foam pads. It is usually much stronger than an APC. It is powerful (bleach) and you will need to rinse it really well afterwards. Probably would not use it on my good MF towels.
Probably more info than you want but hopefully this will give ideas to others.
What would be the most suitable product/s for cleaning grime under the door sills?
OPC, Tar X, P21S Paint Cleanser,...?
Most of the grime comes off with my mf mitt but it's getting grime stuck in it and I know it's going to go hard if I keep on soaking it in APC. That mitt is now only for under the sills, I wouldn't use it on the sides of the sills. So it seems like I need to use three mitts - one for the lower panels and that grimy one for under the sills.
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