What's the secret/trick to skim coating panels
It sounds like your skimming techniques are OK. It could be that your problem is not narrow low spots - they are actually a highs in the sheet metal. It could be that the metal is stretched and it needs to be dealt with before applying filler. If that's not the case. Then as suggested by other replies start with 36 grit on the DA then move up to 80DA. For the next stage, I would not use a Durablock or any flexible sanding tool to leveling the body fillers, you should be using a hard hand sanding 'board' that's the only way to level body fillers, in my shop. Change your sand paper often - keep it fresh so that it cuts down the highs and apply more filler in the lows, if needed. Remember that fillers and primers sand different, so you want to get the whole surface in the same substrate.