Renaults don't usually die of rust.

The design and build quality of the body is far better than the electrics...

SO unless you plan to keep the car more than a decade, it's a waste of money.. in my view.

You could put your new motor up on a hoist,blocked up with all wheels removed.Take off plastic wheel arch inner covers and door&boot trim,then waxoyl the bottom inner of the doors and underneath the arches before replacing all panels.If there are any rubber inspection plugs to gain access between the outer&inner sills you could spray waxoyl in there too,but that may necessitate removal of carpet/trim/seats to gain access to these. Easiest job is external lube of all metal brake and fuel lines and fuel tank,if metal.Use the spay technique so as to lube concealed pipe-work etc.Then carefully blast the whole underside of the car avoiding rubber items and disc brakes/flexi hoses.

Brake calipers will require cleaning and sliding areas lubing with copper grease when the pads are changed.

Never had any problems in the `90`s&`00`s with corrosion on various Renaults and other French cars I`ve owned or MOT`d apart from the myriad of thin pipes that were part of the suspension and braking system on the Citoen BX range,but that could have been avoided if they`d been lubed from new.

Not so in the mid-eighties when every time a French car came in for welding I dreaded getting the mig out.They all spat back like crazy,reckon the steal they used in the`70`s had every type of metal chucked into the melting pot.

Failed a 1990 Alfa Romeo at it`s 1st MOT on corrosion in 1993....can`t remember exact model.Hopefully they`ve improved too.

Thank you for taking the time to reply to our query on rust preventative treatment on the Laguna Coupe. We will show your comments to our trusted mechanic at the end of the month!