If you need extra support for your arm while painting, try a mahl stick. It's a simple tool artists you can either buy or make yourself.
The Mahl Stick - Sign Smith Sign Painting
How to hold a lettering brush and mahl stick : Robertsign
The use of a Mahl stick prevents smudging surrounding artwork while providing a brace for consistent steadiness and balance. This is especially useful
USA Made 36 inch 3 Foot Mahlstick Maulstick Maalstok Painter's Stick Guide Rest
Maulstick - Wikipedia
Made of varnished aluminium this folding mahlstick has traditional leather covered cork end rest (not cheap rubber tipped end). Leather will not slip like rubber can. Very light, very sturdy and certainly the best. Expands to 90cm in length.
Creative Mark Professional Mahl Stick for Painting, Drawing & Sketching Classic Traditional Artist's and Sign Painter's Tool- Expands to 35.5 in
Miniature Painting -- Art Glossary Definition
Taking The Mystery Out of Mahl Sticks - Teresa Bernard Oil Paintings
If you use a mahl stick! - WetCanvas: Online Living for Artists
Painting Over an Old Unfinished Oil Panting
Mahl stick hi-res stock photography and images - Alamy
Studio Equipment: Mahl Sticks
Miniature Painting -- Art Glossary Definition