Staying top of mind sounds like a marketing buzzword. In practice it's simpler: when someone finally needs a weld, yours is the name they think of first.
You don't get there by shouting louder. You get there by showing up regularly, in a small way, with something actually useful.
A few easy things to send
A photo of a recent job and a sentence about it. A seasonal reminder — gates and fences take a beating over winter, so book repairs early. A short answer to a question a customer actually asked you.
A small next step
Write down three things you could tell a past customer that they'd find genuinely helpful. That's your first three months of staying in touch.