Removing old sealant from wood
If it's outside, I'd say it's more than likely some kind of varnish. I think you'd need a "chemical stripper" (there are "green" ones these days: https://www.homedepot.com/p/EZ-Strip-32-oz-Paint-and-Varnish-Stripper-ESA-700/205725398). 

The equation is basically:


Remove shellac with alcohol
Remove lacquer with lacquer thinner
Remove everything else with stripper