A1 and where to get it
In our line of work, it's probable that paint inspector gets involved in certain bureaucracy when moving within the EU.
Such one is an A1 document, formerly known as E101/E103.
This form declares your contribution to the social security system in your domestic country if you have a connection through your employment (also self-employment) with more than one country. It's issued by your social security institution you are registered with.
Find list of contacts for your country here -> http://europa.eu/youreurope/citizens/work/social-security-forms/contact_points_pd_a1.pdf
Since it takes quite a while and some of the possible employers require to have it beforehand you need to dedicate enough time, usually couple of weeks. Sometimes it's possible (e.g. if you were denied access on site) to contact your social security institution and ask them to process your application preferentially.
If you are going to work temporarily in Belgium, you have to inform Belgian authorities before the employment starts. Fortunately, it's possible online through Limosa website. -> http://www.limosa.be/