My personal experience is that search engines in most of the forums I visit are useless; so I use Google instead.
Just in case you do not already know about it, Google has a mechanism to restrict a search to a specific site: all you have to do is add "site:your-site-here" to the search bar; for example, to search for "coating" or "spraying" here, I would go to Google and type "site:www.luminous-landscape.com/forum coating spraying". You are not likely to find less pages using this method, but I find Google's page-ranking mechanism very useful.
Hope this helps.