If you plan to continue working in houses like this, yes, it's worth developing this skill.Look online for how to use oxalic acid and peroxide cream developer to lighten floors, the actual technique you'll use.

Isn't that one of the lead generation techniques? 

If you choose to use this technique these are some of the Pitfalls to watch out for.Subject To Pitfalls1.

I know how frustrating it is to work so hard for something only to lose it, the knowledge or techniques to become out of date and to feel that you have lost your 'edge'.

Multi-family properties allow you to employ several different investment techniques.

This is more a matter or technique and quality of workmanship than anything else. 

Study up on tenant screening techniques, and implement them diligently.CRITICALLY IMPORTANT: Too much noise & too much mess are the two most common problems you’ll have with housemates, so be pro-active in preventing those problems before they occur.

Both drywall and paint are very easy in theory/concept, but hard to get right if you don't have precise knowledge of the techniques.

@Rose Ganor No... you pretty much got it...You could try some of the "influencer's" techniques, and similar techniques taught by real estate investors. 

These techniques, while still doable today, had its heyday in the 1970s and early 1980s when double digit interest rates combined with no due on sale clause accounted for the majority of real estate sales activity.  99.9% of loans are such that the lender wants to know the source of the down payment.