Hi,
I’m a beginner to Rhino and I’m having trouble converting/merging surfaces into a solid so that I can create an .stl file for printing.
Here’s what I have at the moment…
A screenshot doesn’t help us much in determining what’s wrong… Are they separate surfaces? Have you tried Join to join them? Post the file if you can’t figure it out.
–Mitch
Apologies, here is the file! 
Yep, they’re separate surfaces.
Tin 1 print.3dm (102.5 KB)
Just window select all and choose “Join” (puzzle piece icon). Done… --Mitch
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thank you! I’m such a beginner!