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I have just bought a Black Mountain spring loaded thumb pick which I will be trying out as a relative newbie. I find thumb picks quite comfortable but struggle to find one which sits still without moving around a bit so thought I would try the Black Mountain one out. Will report back with my thoughts.
For anyone starting out with a thumb pick I recommend the Herco Flat Thumb Picks:https://www.stringsdirect.co.uk/accessories-c2/picks-plectrums-c50/thumb-picks-c409/herco-flat-thumb-pick-heavy-3-pack-p10853?gclid=EAIaIQobChMIjd3q1cvh6gIVD-ztCh3t_gqYEAUYASABEgIGM_D_BwEAs a mid way between a plectrum and a thumb pick they are easy to adapt to. They come in light, medium and heavy gauges but I suggest heavy. To stop thumb picks turning while you are playing just moisten your thumb before putting them on.