What is the best way to put a single to double concave into a blank?
take some sandpaper…put in your single…blend it out to a double…have a beer.
what i like to do is start my single concave aroud 4-8’’ of nose and double around 24’’ of the tail depending on the board plot it out and use a shureform the a foam sanding block to smooth it out.
Personally, i bring the stringer down with a block plane to the desired depth, then smooth it out all the way to the rails using a sanding block and the sureform. then i use the sureform on eitherside where i want my fins to be and bring it down another quarter of an inch or sometimes more, and then repeat the smoothing out process, but the concaves end further away from the rails, then i use the sureform to blend it as best as i can, then use some 150 grit to blend it even more. Thats what works for me.
For the single use your planer for the single to take down the stringer to desired depth then do progressively shallower cuts out to the rail, blend with sureform then sanding block. For the double I use a foam pad with a screen and sand it in by hand.