I design instrumentation electronics that use relays. Sealing isn't always the best if the relay wasn't designed for it originally. Sometimes the internal plastics and potting materials outgas and foul the contacts. It's not usually a problem for power relays, but it can be if they don't cycle often (cycling scrubs the crud off the contacts). I would be wary of a no-name Chinese relay manufacturer. We have to do pretty extensive reliability testing for our instruments, and even relays from the name brand people have problems.
