@J as fediverse token christian: honestly go for it
if you really want to mind your ps and qs, most churches have an office or admin section that is open all the time - pop your head in, let them know what you're doing, or give them a call. if you're wanting to draw/paint inside this is a bit more of a required step, but outside is fair game.
expect some friendly and polite curiosity about what you're doing, and invites to come in for a cup of coffee/etc lol
@J some churches do have services etc. on weekdays but generally it ends up being on wed around here not monday, and it's more likely to be activities (midweek bible study, etc.) than straight-up services, so not the typical crowd of people filtering in and out
basically treat the space politely, take the boyscout mentality of 'leave your camp cleaner than you found it', and you'll be in the clear to paint the outside for sure
