El pegamento no seca rápido; mejor usa cinta para cerrar el sobre.

Both are used. Strictly speaking:

• Transitive: secar = “to dry (something).” Example: El sol seca la ropa. (The sun dries the clothes.)
• Pronominal/intransitive: secarse = “to get dry.” Example: La ropa se seca al sol.

With substances like paint or glue, many speakers also use intransitive secar without “se”: La pintura/El pegamento seca rápido. This is widespread and accepted. However, se seca is more neutral and common everywhere: El pegamento no se seca rápido.

It’s an ellipsis meaning “(It’s) better (if you) use…,” so it’s a mild, friendly suggestion—like “You’d better use tape.” Softer or more explicit variants:

• Es mejor usar cinta…
• Sería mejor que usaras cinta…
• Más vale que uses cinta… (slightly stronger warning flavor)

With affirmative commands, pronouns attach to the end; with negatives, they go before:

• “Use it (the tape).” → Mejor úsala (because cinta is feminine).
• “Use tape to close it (the envelope).” → Mejor usa cinta para cerrarlo or Mejor úsala para cerrarlo.
• Negatives: No la uses, No lo cierres. Note the accent on úsala/úsalo/úsenla, etc.

Spanish often omits the article when referring to materials/tools in a generic way: usa cinta ≈ “use tape.”

• la cinta = a specific tape already known in context.
• una cinta would mean “a (single) roll/piece of tape,” which is unusual unless you’re emphasizing “one piece/one roll.” More natural: usa un pedazo de cinta if you mean a piece.

Cinta can mean tape or ribbon. If there’s any chance of confusion, say cinta adhesiva (adhesive tape). Other common specifics:

• cinta transparente (clear tape)
• cinta de enmascarar (masking tape) Brand-based terms exist (e.g., “Scotch,” “Diurex” in Mexico), but cinta (adhesiva) is widely understood across Latin America.

The semicolon neatly links two closely related ideas while keeping them separate. You could also write:

• Comma (common in informal writing): …, mejor usa cinta…
• Period: El pegamento no seca rápido. Mejor usa cinta… All are acceptable; the semicolon is just a tidy stylistic choice.

Be careful—word order changes meaning:

• Mejor usa… = “You’d better use…” (a recommendation)
• Usa mejor… = “Use better…” (i.e., improve the way you use something), or “Use better tape” if followed by a noun: Usa mejor cinta (comparative of quality), which is not the intended meaning here.

Yes, but usage varies by country:

• pegamento: neutral and widely understood.
• goma: “glue” in several countries; in others it means “eraser.”
• pegante: common in Colombia.
• cola: often “wood glue”; more common in Spain.
• Brand-based: Resistol (Mexico), etc. If in doubt, pegamento is safe.