I modified this old card so I could spread paint on the inside of my boxes. I trimmed two of sides at angles so I can spread paint on surfaces that meet at 90° angles without scraping wet paint off the sides.
Plastic card with the sides trimmed at angles.
When I’m swiping down to spread paint the card won’t scrape the adjacent vertical surface (now and then that happens by mistake). The points of the card are sharp so they fit in corners.
You can see that this unmodified card would scrap the newly painted sides of this box and the points of the card are rounded so they don’t reach into corners. I mostly use these cards for spreading on open surfaces such as cradled wood panels.