cuprinol spray and brush preserver

Hi, we have put up a new wooden fence, how long should we wait before we put on the cuprinol fence preserver; if at all ?
is cuprinol the best way to go? when i was in B&Q i saw their own brand preserver was cheaper. also both their brand and cuprinol do a spray on version. i worked it out, the spray on cuprinol cost 57 pence a square metre and the brush on cuprinol 30p a metre. why the big difference, are they different in any significant way i ask?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file
Upload is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.