General practitioners (GPs) are encouraged to offer weight loss advice whenever appropriate to patients living with obesity in primary care. However, conversations on weight present interactional difficulties for both GPs and patients and hold the potential to reinforce weight stigma. Existing guidance offers little support on how clinicians can use language strategically to navigate the interactional difficulties of discussing weight opportunistically. This study examines how GPs manage the challenges of weight-loss advice delivery during routine consultations. Drawing on Brown and Levinson's (1987) politeness theory and using discourse and conversation analysis of 185 audio-recorded GP-patient encounters, we show that clinicians employ both negative and positive politeness to enhance the acceptability of advice. Negative politeness strategies - such as agency mitigation, reducing the illocutionary force of the utterance, and point-of-view distancing - help soften criticism, avoid explicit allocation of blame, and protect autonomy when broaching the sensitive topic of weight. Positive politeness - attending to patients' interests and wants, exaggerating approval or interest, presupposing cooperation, and including both the speaker and the addressee in the projected activity - works to build rapport, support cooperation, and facilitate commitment to next steps. The use of positive politeness when suggesting further actions was associated with better patient uptake of advice and facilitated closing the sequence on a positive note. We argue that using politeness strategies in sequence - starting with negative politeness to broach the topic and moving to positive politeness when discussing next steps - allows GPs to navigate these sensitive conversations more smoothly. Strategic use of politeness has the potential to reduce the risk of reinforcing weight stigma in primary care.